First, to address the main point: I like grim and gritty pseudo medieval campaigns. That's my goal.
No plane hopping adventure is consistent with that stated goal; accordingly, picnics to the lower planes is, to me, anethema to the campaign feel I want to run or play in.
Second, there is an inherent bias in the question, as follows:
"Why do most groups avoid DragonLance?"
<insert recent spew of invective over railroading and sterotypical kender>
"Why do most groups avoid Eberron?"
<insert less recent spew over lighting rails and warforged nonsense>
"Why do most groups avoid Greyhawk?"
<insert debate over old greyhawk, new greyhwak and whether most home brews are essentially Greyhwak in all but name, so that, in fact, we don't avoid it at all. Meh>
"Why do most groups avoid Frogrotten Realms?"
<insert debate over whether that is in fact true. Grudgingly admit it may be so, but that's its still hugely popular all the same and screw you if you don't like it.>
"Why do most groups avoid Darksun, Spelljammer, etc etc etc et al."
<insert etc.>
My point is: we can say with a straight face and with a plausible streak of sincerity that
most groups avoid
everything.
The RPG community is so fractured, there is no sense of normalcy or "default" style, no matter the example you choose or the campaign name or style you wish to insert in the question.
To those who took offence at the Planescape digs, *shrug*. It's just your turn this week. Next week it will be somebody else's
