Because they are silly and ridiculous in a game setting where players generally try to take the fictional world seriously. It was part of an unfortunate trend at the time to put things that were generally considered "silly" into otherwise coherent milieus - the greater outrage was a year earlier when WG7 Castle Greyhawk turned out to be a huge megamodule that was 100% joke content, when people had been waiting for a decade to see Gary's real Castle Greyhawk. That silliness was taken as being quite insulting to the gamer community.
When Spelljammer came out the giant space hamster seemed like more of the same - although some people really liked that, the patently ridiculous nature of the GSH rubbed many fans the same way. (The GSH appears in dozens of online articles entitled things like "X Most Stupid D&D Monsters"...)
Since Gary Gygax had been forced out of TSR a couple years earlier, the GSH was, justly or unjustly, seen as a minor symbol of the decline of content quality (even though Gygax penned otherwise somewhat silly content like Land Beyond the Magic Mirror and Dungeonland) and related perceived contempt for gamers under the new TSR. The GSH therefore got caught up in the larger community issues at hand, so while at other times a joke monster might just be ignored, at this juncture it generated vitriol instead.