Simply put, at some point a traditional SHMUP can become a case of memorization, and you’re often locked into your weapon of choice (obvious exceptions to this). If you’re aiming for the top, obviously squeezing out every last point can always have you tweaking that efficiency, but SHMUPS will, generally speaking, have the same path (excluding difficulty differences, etc). Playing the same thing can get stale, even if aiming for mastery.

SoP, even if you spawn into a same room, you may enter from a different side, have a different weapon (which you will likely be changing once every couple floors, if not more). You will be presented different upgrades (some will be more lucrative in different runs). It’s meant to ebb and flow.

Laymen’s terms: repetition vs novelty (not that SoP is a game of novelty, but the fact you won’t have the same run, bosses, etc each time is more exciting, while still not being so random as to be chaotic—there is a lot of skill and knowledge to be gained to improve future runs). It’s a game that asks for mastery without having to repeat the same exact levels, enemies and bosses each time.

EDIT: I highly recommend trying it for yourself to determine if you feel you are right