Because they're way too big, plain and simple.
The collectible ones are IMO the keyboard models: A500+, A600, A1200.
A500+ is the good ECS one. With 1MB chip and kickstart 37 pretty much guaranteed. I don't own one, but I have an older A500 with the little mod to get 1MB chip, which is almost as good.
A600 is the "bring along" one as it's smaller. Supporting IDE HDs is obviously very convenient. Kickstart 37. Real problems include hardware tending to break more than A500+ and software requiring numpad, which the A600 lacks. Lack of expansion options used to be an issue, but there's some interesting hardware now, such as a crazy fast (relatively) FPGA based accel board.
A1200 is the option with AGA. I own one paired with a 68030 accel board. Together with whdload it's godly for playing Amiga games on the real hardware.