There are a lot of series like this that run on for a long time with very simple premise, let alone those that actually try to develop into something more complicated down the line. The premise of him being the only boy in a school was enough to get things rolling and the popularity must have been steady to keep the ball rolling as long as possible.
They've seemingly stopped introducing new girls, minus that random girl at the fashion show that'll probably appear again, so now we are at least in the full pursuit phase.