A contract ran out, it wasn't them pulling it off maliciously.
Same reason why there was no YouTube.
Do we know why negotiations failed?
As I understand it, Google refused to allow Apple to use Turn-by-Turn directions in their version of navigation. Basically, if Apple wanted to provide TBT to their customers (something Android people LOVED to rip iPhone owners about), they had no choice but to develop their own Maps application.
...Now whether or not it was wise to pull Google Maps before Apple Maps was fully tested is another discussion (for one of the 650,000 existing threads about it)...