App Engine is far more mature than Parse - it's been around for nearly a decade, the API is stable, and Google's not going anywhere any time soon. App Engine adoption was also very slow to pick up, but it was a more enterprise-targeted computing solution than Parse.

IMO Facebook just decided to cut their losses. Parse was very likely to end up as a low-margin enterprise business with little to no impact on Facebook from a data standpoint. Facebook doesn't really have an enterprise sales division like Google does; and standing up a new sales channel like that is very difficult if you're not sure the margin will be there.