I'll tell you why:

Because it, kind of like Brokeback Mountain all those years earlier, appreciates that homosexuality and the gay community are not one in the same. While the gay community is extremely important and should never be downplayed, media always tends depict gay people as connected to it or at the very least evoking many of the same aspects and tropes.

This isn't an unfair thing to depict, far from it, but it has the unfortunate result of making many gay characters feels rather same-y, occasionally even one note.

Offerman's character depicts a very accurate thing that doesn't get as much attention in media: the straight acting man discovering his sexuality late in life. With the exception of his piano playing and his penchant for wine and setting the table for elaborate dinners, his character has none of the telltale "gay" aspects you typically find in media, nor does he develop them. In fact his character aspects (survivalist, paranoid, shut in, loner, even a hint conservative) are generally not associated with gay characters, out of fear of depicting them in a poor light.

That's not at all to suggest other depictions of gay men are wrong or bad, Bartlett's character is very well done too. The characters of Bill just feels more notable and fresher in our current media landscape because we see it far less often.

I got to see his standup routine a few months back and it was hilarious and wholesome.