The seven year thing is probably more coincidence than an actual pattern.
George wanted to release the SE in '96 and TPM in '97 until plans changed.
He wanted to wait until RotS was finished to release the OT on dvd … until he realized fans had waited long enough.
As for the timing of the blu-ray, well, there was no new movie release to time it with, just the ongoing clone wars show. They didn’t want to wait forever like they had with the dvd release and probably just said “screw it, now’s as good a time as any.”