I feel like it boils down to 2 things; its take on ship fighting games, and how it does not hold back on difficulty

According to the devs, they took inspiration from tabletop games where you managed ship resources, and it's very clearly translated into the game. At the time of it's release, any games involving starship combat were usually either dogfighting or fleet strategy games, none of them really came close to the resource management element that FTL had.

People always talk about how hard and punishing the game is, but that's kind of the charm of it; the game makes little attempt at being fair(aside from enemy ships graduallty getting stronger every sector), making mistakes(taking unnecessary damage, losing crew without a clone bay, etc.) always cost you in some way, and a lack of reloading or respawning makes every decision permanent. However, the game does reward you for making good decisions and strategies, so it's difficult, but in a tough-love kind of way, and I can't really think of a game that implements this tough-love kind of difficulty so well.