Basically the problem is that you have to buy second hand hardware, and then it'll probably work. People tend to assume that because it runs on a PC, that it'll run on a recent PC - but that's usually not the case.
That's not even close to being true.
If you said "outdated hardware" you might be closer but MorphOS and OS4 only run on even more outdated hardware.
Yet AROS runs on top of the line Intel and AMD CPU's both 32bit and 64bit, also various 680x0 ARM and PPC CPU's, has full hardware accelerated 2D and 3D graphics on the best hardware money can buy, full USB2.0 support, proper DMA, runs hosted on top of other Operating Systems as varied as Android, BSD, Linux and OSX without depending on emulation, runs natively on genuine real Amiga hardware, I could go on forever.
I love my MorphOS boxes as much as anyone (1 Pegasos, 2 PowerMacs and a PowerBook on its way for my birthday present) and you'll have to prize my real Amigas from my cold dead hands (I even used to run OS4 on a BPPC but it wasn't a pleasant experience but I'd gladly run it on a faster hardware should it ever be ported to something that isn't overpriced junk) but I don't live in cloud cuckoo land, its blatantly obvious that AROS supports far more hardware than the Amiga OS or MorphOS OS4 do combined.
It is also the only realistic future for the platform in the long term due to its Free and Open Source nature. When the MOS Team get bored/die of old age and Hyperion run out of money, the AROS code will still be out there on the interwebs for anyone who wants to put it on new hardware.