Three reasons why I felt the Amiga never truly caught on.
Lack of marketing: Marketing touting the c64 was everywhere, but when the 128 came out, it wasn't given the same flash as it's preprocessor. There wasn't a push to say "the amiga is awesome enough to make you want to drop your reliable c64".
Gaming consoles: One of the things that I remember the c64 having as a strength was the quantity, and quality of it's game selection. By the time the amiga came out, the second console war was starting up again with Sega giving nintendo a run for it's money, and with both ramping up their products and the game selection, developers followed the money.
IBM clones becoming more affordable: Around the time at the Amiga was out in full swing I recall the prices for IBM clone systems falling to levels that an average middle income family could afford. Sadly I dont' have data, but I do recall that we went for an IBM system over an amiga, and price was usually the lone factor in deciding things in my household so...
Now granted this is after almost two decades of time that has passed but that's the impression that I always felt was the reason why the amiga never truly caught on.