It depends. Some of the artists declined in esteem as people took greater notice of their flaws (e.g., disproportionate anatomy in Liefeld's art). Some of it is changing artistic tastes. Some of it is personal taste (sometimes people use the word "hack" to mean "artist whose work I don't like"). Some of is blaming them for the speculation bubble, whether that's fair or not.
I think a lot of it is frustration with repeated publishing delays. And I think a lot of it was a feeling that the stories suffered when some of the artists took over writing duties (either on their Marvel books or when they started at Image).
Personally, I think there are plenty of artists whose work actually gets weaker over time. So some of the artists may have been better earlier in their careers.
Not always true. There was a drawn-out legal fight between McFarlane and Gaiman over ownership of characters, whether certain characters were derivative or original, etc.And they got paid better, by many accounts, than they did for their DC and Marvel work, and owned things they made up for those stories.