I think it's for a few reasons:
1) because it was a really powerful system that was just under the complexity curve such that a dedicated person could learn literally everything about the system from top to bottom and do cool stuff with it.
2) It's stock form was insanely powerful out of the box, while PCs were from different vendors, with different configurations and no particular standard "thing" was a PC.
3) It wasn't really meant to be tinkered with, but almost everybody who had an Amiga had done some crazy things to upgrade it. The hacker community on it was vibrant and technically awesome (see #1 above). Whereas if I wanted to upgrade my PC I simply inserted a new card or something, these guys were soldering new OS ROMS and SCSI interfaces into a system that really wasn't supposed to have these things. I remember reading about Power PC expansion boards for the A500 and wondering "where's the expansion slots?"
4) Because of #3, systems took on very individualistic flavors, like muscle cars tricked out by their owners. You knew that a guy with an A500 that had the latest OS ROMs, two hard drives, a VGA monitor, a couple of floppy drives and was on his 3rd keyboard had put a lot of personal time into the physical aspects of his box. The same couldn't really be said (at least not to anywhere near the level) with PC owners.
5) Amiga user group meetings were a blast with people bringing and showing off their machines, trading pirated software (anybody remember "Fish disks"?) and generally have a great time trading trick out secrets with each other. It felt like a car club community event rather than a computer community event.