The match fixing scandal in the Brood War days caused Starcraft's popularity to decline massively. Also, Starcraft 2 wasn't as well received due to things such as Blizzard and KeSPA having disagreements about how to run the competitive side game, SC2 being online-only, pay-2-play (at the time), the pushed for the ranked ladder which lead to ladder anxiety for players, and the RTS genre as a whole was falling off in favor of other genres such as MOBAs. The only other RTSes you hear about these days is Age of Empires 2 (which is still getting updated and has a small, yet dedicated community), and Warcraft 3 with the upcoming remaster of that game.