Those services are seldom profitable. Especially as they get larger, their costs rise. Meanwhile, imgur, as a service that provides embedded content, has little opportunity to make money off of their users. They rely on infinite growth and ever more people investing money into them to keep financially viable.

But there is no infinite growth and imgur has reached its limits. Now they need to bind users to their platform and rely on ad revenue. So old content gets purged, along with nsfw content, in order to entice advertisers.