I agree with a lot of the posters here as to why the Ultras are quite so hated.
They are very highly exposed, they are pretty much the poster boys of GW and WH40K.
They are presented as THE Codex chapter, the shining example to all the rest of the chapters.
They are Peter Perfect, who can do no wrong, flawless in every way, and that just grates on some people.
Not to mention that I wouldn't be particularly tempted to play Space Marines in ANY form, even if it was a very obscure chapter, due to the simple fact that they're everywhere. There still is a tiny inkling to get some, maybe a very unknown chapter or an original one ("The Space Ponies" for example) and not use any special characters, but really I'd rather collect an underused army.