This is how I remember it: For some time LEDs were red, yellow or green. Power LEDs were almost universally red. You can see this on devices from the homecomputer era (Amigas, Ataris, etc.). I'm not sure if red was chosen for technical reasons (red LEDs have the lowest voltage drop), maybe economical reasons or it was already a convention before LEDs became available.

Anyway, when blue LEDs became feasible they were the epitome of cool and every device had to have them. So in my opinion it was a fashion trend that stuck.