Ordering people to write a certain way tends to kill their muse. This is because everyone is called to write different things, which is good. Don't ruin the fun.
Also, intruding on someone's creativity is a good way to make them hate you and/or your goals. Which if your goal is diversity, that is the opposite of helpful.
Implying that something is wrong with everyone who doesn't write the way you do is pretty snotty, too. See above re: people writing all different things and the desirability thereof.
Demanding that people write things they don't want to write, if they follow through, typically leads to lousy writing. Seriously, look at all the shitty representations of women, people of color, etc. stuck in there because someone said they had to be, not because the writer wanted them there or the story needed them there. Do not make tokens pour out of the tokenism machine. Argh. No. A decent editor will just cut that crap anyway. Unless they commissioned it, in which case they are idiots, because if you want good copy then you assign it to someone passionate about Topic X, not someone who finds it boring or distasteful.
As much as it may embarrass modern, progressive folks ... a lot of history was pretty narrowminded. For a long time, the vast majority of people stuck very close to home, which meant there wasn't much diversity throughout large swaths of time and space. This is likely to repeat itself in speculative settings. That doesn't mean, for instance, there were no gay people but rather than they tended to hide for sake of survival. You didn't get a choice of religions if there was only one around you, unless you happened to have your own link to the Divine and/or packed your menu into this life via Farmemory. If you want to tell certain types of stories, you need an iconoclastic background; and if you want certain settings, you have to deal with the fact that they were boringly homogenized in certain ways.
Some other parts of history were really diverse. They often didn't get along with their more isolationist, monolithic neighbors. If you want to write that, great, but be prepared to do a lot of research -- or worldbuilding, if you're setting it in some other world. Because diversity just doesn't look the same every time you make it. People didn't always think about race, religion, sexuality, gender, etc. the way contemporary culture does. Which is actually pretty cool and a reason to write history or fantasy stuff in the first place.
Finally, unless you are paying for the privilege of telling someone what to write, you don't have it, so STFU.
Me, I like diversity, but that doesn't mean I put it in everything. Especially, trying to put ALL the types of diversity together is difficult to do well, unless it is 1) a really big work like a novel or series and/or 2) the kind of setting which is diverse by its nature like an an interdimensional hub. Big issues need plenty of room.
A Conflagration of Dragons is really about two things: race relations and disasters, and following from those, how race relations influence the way people respond to disasters. They all had contact with each other -- except that the dragons just woke up from lengthy hibernation -- but they didn't actually live together very much. Until they stopped having choices about that, because refugees have to take what they can get.
Diminished Expectations has a ton of diversity in body shape and even species, counting the created beings. But it's still a craptastic place to live. Being diverse doesn't necessarily make a society a nice place to live. I think this series has maybe 3 fans.
The Ocracies is a setting that I literally made to play with diversity, specifically in politics. I got bored with all the McMonarchies and started making up scads of little countries with all different governments. Other types of diversity exist in the setting but are largely incidental to trying to show how all these wacky systems could actually work.
Fiorenza the Wisewoman is one of my historic ethnic series, and it's fairytale Renaissance Italy. There's a little bit of racial diversity, but most of that happens on market days or in a city, because backwater Italian villages of the time weren't very mixed. The only religious diversity they really have is the fact that Italy is a palimpsest of old and new traditions, but it's not actually all that mixed because most characters go to church on Sundays and drop offerings at roadside shrines while on the road -- not two different groups each with its own religion. There's a little bit of sexual diversity but it only comes up in a few poems. Most of that just comes from Fiorenza and Giacinto each being just a hair off from strictly feminine or masculine. So there's a dab of diversity, but it's not really what this storyline is about.
Beneath the Family Tree (on the Serial Poetry page) just kind of smears over the whole issue of diversity. They have three possibly different species, possibly quite divergent races who wound up living together just find and not really making a big deal of it. They don't care about sex/gender diversity either. Gullwing seems unattracted to male bodies but happy enough with Cobble, who insists that he is a man instead of one-between. Nobody else is exercised about any of that, particularly once the two settle down together. Is it still diversity if people don't care about it the way we do? Is it even background parity? To me it just feels pre-differentiated.
The Origami Mage is among my least diverse series. It's set in a fantasy Asia, so basically everyone there is Asian, because Asia has had a lot of its cultures go through very isolationist phases. Also the story is very inward, it's about Asian motifs, and mixing in other characters would just be a distraction. It's not big on exploring sexual or religious diversity either. It's about a rather fussy little division between how two young women work paper magic.
So there's a spectrum, and it depends on what I want to write about and where. I love diversity. That doesn't mean it's the only thing that interests me.
By all means, encourage people to try writing about more diverse characters. Prompt for it. Shop for it. Create resources to make it easier for writers to do it accurately. But don't try to force them. You won't make any allies that way, and you certainly won't make good literature. Don't be a dick. Tell ALL the stories.