Does this question really need to be asked? Why would someone have a huge gas guzzling suv when you could own a honda civic that's reliable and good on gas? Why would anyone own a honda civic when you could have a ford mustang that's fast and exciting. Why would anyone own a ford mustang when you could have a subaru legacy wagon that's practical and good in the snow? Why would anyone own a subaru legacy when you could have a mclaren f1 that goes like a rocket ship and will get you chicks?
There are any number of reasons why not every single person out there wouldnt have the same type of amp.
#1 People just prefer different things.
#2 For some people, weight/size isn't really a concern for whatever reason.
#3 Some people might just be happy with what they have already.
#4 Some people might have the money to go out and buy a brand new amp, especially if their current one (or ones) is/are working fine.
#5 When people do buy the micro-amps, they usually sell what they had before, creating a buyer's market for the "older" amps.
#6 Some people don't buy into the overly consumerist society we live in where you always have to have the "new best thing".
#7 The environment really doesn't need every single player out there buying a new amp every year or two.
For me personally, it's a little bit of all/most of these. I like a mix of old school/modern. My cabs/speakers are 20+ years old, my "backup" head (since I got a new one recently) is over 15 years old, has a tube preamp 400w @ 4ohms, is 2 ohm stable (450w) sounds great, and weighs about 16 pounds. I paid about $375 for it in excellent condition a year ago. So what would I gain if I sold it in and spent more money on a "micro" head? I'd save maybe 5-10 pounds, and a bit of space I guess. But it wouldn't necessarily have the "mojo" and vibe of a classic amp, and it wouldn't necessarily sound better. And I couldn't hook 4 8 ohm cabs to it! Plus, this one is a brand I had always wanted (SWR) from before they got swallowed by a big corporation (Fender), and the thing is in such great shape that especially considering it was my first real bass head I will probably never get rid of it.
My new main amp I have not because of anything wrong with the SWR, but because it's from another brand that I've always wanted (Trace) and I get a crazy ridiculous deal on it that was good too pass up. This one has about 1/3 more power than the SWR (not 2ohm stable though, according to the manual, although the thing is pretty bullet proof), is a little more flexible sound wise and sounds EVEN BETTER than the SWR, also has a tube pre with more easily gotten overdrive, a 12 band EQ, dual band compression, and a very comprehensive set of ins and outs. It weighs 30 pounds, but who cares? It sounds awesome and I can carry it by myself and I got it for 1/5 of what it would be new (which it was, basically), so why should I replace it?
Honestly, if I get anything else at this point, it will probably be something like an Acoustic 370. I grew up with amazing high powered solid state stereo gear in the house (McIntosh, with JBL speakers), so I don't have the aversion to it a lot of guys have and I actually like that sound, especially of 70s solid state stuff that's really well built.
Eventually I'd like a medium/high powered tube bass head too, either Ampeg or Trace or a Hiwatt clone, something like that. But until then I have an old 70w Bassman and a Musicman 65 (tube power amp, SS pre) that will get the job done for recording purposes and screwing around.