there's such a variety of "adventure" bikes because everyone's idea of "adventure" is different. to some it's crossing the world or large chunks of it. to some it's just going down a road they don't know and finding out where it comes out. Some its gravel roads, some want single track.
Most of my friends have gravitated towards smaller bikes the past few years, mostly WR250r with some well placed mods. But that's for some trips they were doing out west. They would trailer out and ride the fun stuff on the WRs and had a blast.
The big advantage for some is having the right bike for the right type of riding you want to do. When friends ask me for bike advice, my first question is who will you be riding with? what bikes do they have? that's the direction to follow. because if you want to ride with them, they want to ride stuff that's appropriate for their bike. if they have KLR's don't buy a KTM 500 you'll only get frustrated on the paved or smooth gravel roads the KLR's are wanting to ride. If they have KTM 500's don't buy a KLR as you will be miserable riding single track and picking up your KLR.
everyone has touched on what they feel is important to the buyer- comfort for long range, ease of maintanance, durability, ability to find parts in other parts of the world, handling, weight, seat height, fuel range, etc. etc. etc. The list is endless. that's why there are so many "adv" bikes out there. something for everyone. I have a tagged 450 and it's fun in the right environment but when few DS rides I did I hated. 5% of fun stuff on the 450, 90% boring riding. I had to do stuff to make it fun. I've also been on rides with people on everything from a 200 to a 1200. everyone had fun, just some liked different parts of the ride differently. :)
Now that I have a 950, it makes those rides fun again. the smooth stuff is easy to ride, but the middle ground is more fun, and the 5% stuff is even still fun (for me!). after riding the past two weekends, I think it's the best of both worlds. I won't take it down super technical singletrack but just about anywhere my friend will take his 690 I'll take my SE, but the pavement portions of the ride are probably more fun for me. Even on some tight twisty roads this past weekend, it almost felt like a decent SM (minus the trail braking on knobbies).
different strokes for different folks...