But they don't become memorable and enduring instantly.
People usually latch onto the background characters because of an interesting design (Vinyl's hair, Derpy's eyes), or some kind of quirk (Bonbon's voice) and then they create the 'rich back-story' and 'unique character traits' themselves.
We like the background characters not because they are amazing, fully developed personalities, but because they are (mostly) blank slates that are open to interpretation and expansion.

I kinda wish that they would do the above with more of them, though the whole "fan name vs "real name" issue might lose a bunch. Kinda glad that more of the background/ so far single use ponies have their names stated in the episode when they first appear to mitigate this.