I bet this is just Apple being Apple. I can’t think of a reason why they’d drop support other than to move people to Photos and paying money for iCloud.
Are you joking? Aperture was great, but it was built on an aging architecture and it didn't provide the automatic syncing abilities Apple understood most users would want from a photo editing application going forward. There also were issues with how it managed its database and whether that could be scaled to accommodate the usage patterns brought about by mobile phone photography -- which Aperture wasn't written for. I hoped they would undertake a ground-up rewrite of Aperture that would preserve most of the user interface, but let's not pretend there was no reason to need to develop something new.