I think a lot of it comes down to wish fulfillment that feels just plausible enough. You know, the idea that someone extraordinarily powerful, who could have anyone, chooses someone regular—or at least, someone outside his elite circle. It’s a fantasy of being seen for your intrinsic worth, not your wealth or status. The dynamic creates instant high stakes and conflict: navigating his ruthless business world, dealing with jealous rivals, the whole fish-out-of-water scenario. Authors also get to play with that classic 'icy exterior melts for the one special person' arc, which is a character beat readers never seem to tire of.
That said, the trope’s longevity might hinge on its flexibility. It can be a fluffy office romance, a dark mafia-adjacent story where the CEO has dubious morals, or a marriage-of-convenience plot. The power imbalance is the engine, but you can tune it for different levels of steam or drama. Lately I’ve seen more subversions too, where the female lead is equally ambitious or secretly holds power over him, which keeps the formula from going stale.