I think "Unix-y machine with decent drivers" is a large part of it. Another is that the battery life is pretty good. Another is that the hardware build quality is reliably good. The main complaint people seem to have is about the aluminium getting hot -- which IMO is far less of a problem than having shoddy hinges, failing keyboard, etc.

It's interesting to see the shift; I know some old-school techies who've moved to Macbooks despite using Linux/BSD laptops for as long as laptops have been practical.