Originally Posted by
Reynolds
Language isn't logical. Or maybe it has a logic nobody was able to understand yet.
Originally Posted by
Cycletography
Generally speaking, the English language has just as many exceptions as it has rules. I gave up asking "why" decades ago.

Basically, it's a hybrid between Germanic and Romance language, with borrowed "rules" getting applied in weird and inconsistent ways. Bicycle is a borrowed French word, and kept a Frenchish pronunciation, motorcycle didn't get into English that way, so kept the English pronunciation of cycle.