Mostly it's bias.
I mean, I can't speak for the original poster, but all I have heard of the 90's is it was a time when comics were demanding to be treated seriously when they were at their most immature, substituting sex and violence for real adult issues. It's also known as a time when pretty art took precedence over good stories, and that "pretty art" ain't so pretty by today's standards.
And the X-titles were the worst of the lot; chock full of bad characters, bad writing, bad plots, and completely impenetrable to casual fans. Rife with villains such as Apocalypse, Stryfe and Onslaught; each more powerful than the last, and all about as interesting as cardboard box. A cardboard box with ridiculous proportions, terribly vague motivations, unless of course it doesn't even have motivations, and armed with whatever powers the writers want to give it this week.
Most of what I heard about the 90's X-books is that they should be killed in their sleep, burned to ash, buried in the desert, and erased from the annuals of history. So you understand when I'm a little wary of 90s concepts and characters coming back.