Considering that the conversion/migration from www.avidgamers.com was to a subdomain of the original domain to ag2.avidgamers.com... it wasn't much of a migration.
It's been 5 years... everything went dark in 2007.
From the few posts I've read where people were asking about the site and where it went/why it went down, there doesn't appear to be any single site that members migrated to. They seemed to drift to many of the other sites that are out there, without any majority going to one single place.
So, no one really took over when avidgamers.com collapsed. Or rather, when the domain expired, it was purchased through dotster.com and hosted on a domain parking service called VooDoo.com. No single site rose to the challenge or managed to attract the community that was at Avidgamer.
The first occurrence of the term "buff" in a published D&D work I can find is the Psionics Handbook (2001) for 3e, where the astral construct has abilities such as "Buff (Ex): The astral construct gains an extra 5 hit points", with "Improved Buff" and "Extra Buff," meaning the term wasn't being used purely in the sense of "strong."
The Wizards forums start to see the term "buff" used later - 2003 is the first occurrence I can find in the Wizards forums, and at the time it is often enclosed in quotes which indicate it's not in completely common usage. Buff doesn't appear in the 2001 "Common Message Board Terminology" list. Similarly, the first use I can find in the context of "'buff' spells" on the RPG.net forums is September 2002.
The next place I find the term used a lot in a published product, interestingly, is in Living Greyhawk adventures, exploding into wide use in season 4 (~2003-2004) - these were written more informally by community members and you can see the term gain great currency there quickly. Not coincidentally, 2004 is when World of Warcraft launched and became a huge fad.
The next mention I can find is in the 3.5e Warcraft RPG Lands of Conflict (2004) where it talks about someone "buffing their skills" - this could demonstrate the use of the common computer gaming term being imported into D&D in its true form for the first time.
Dungeons and Dragons for Dummies (2005) has an entire section on "Buffing: Making your character and the team better." It also states "The cleric's spells revolve around healing damage and enhancing the abilities of other party members. These enhancements, commonly called buffs, allow the cleric to make everybody around him or her better." D&D For Dummies has a lot of sections employing common CharOp terminology and advice from the time - "buff" is used extensively throughout their discussion of recommended builds, including "the buffer sorcerer" et al. The term is used no less than 52 times in this work so I think it's fair to say it is completely established by this time.
So the term appeared in tabletop land in late 2002 and exploded in use over the next couple years. The term was in use in video games prior to that - a mention from Anarchy Online in 2001 for example, and Everquest players were using it in 1999.
Therefore it seems clear that the term was first imported from normal English (buff, to shine up) into video gaming terminology for enhancements and then found its way into tabletop.
Best Answer
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.