Because of Tiamat, Strahd, Acererak, Volo, Eberron, FR, Ravenloft, etc: so many iconic monsters, fiends, settings and what not. Not many other RPGs have so much lore and fluff (with some exceptions like CoC and WFRP).
Because of DragonTalk, Critical Role, How We Roll podcast etc. Not many other RPGs have so many high-quality podcasts. Especially Dragon Talk is amazing. I love listening to the thoughts of Perkins, Crawford, Cernet, Wyatt, Tito, Mazenoble et. al about the game. Such a weekly treat.
Because of the extremely high quality of their supplements. I know people always disagree on it, but personally I find most 5e books to be of a very high quality. And what subjective issues they might have, I can correct when preparing. Few other companies provide same value for money (again there are exceptions like Cubicle 7's WFRP and Chaosium's products - perhaps also Free League and Paizo). A 5e adventure hardback gives me so many hours of playing it's almost a steal comparing to cinemas, novels etc.
Further companies like Kobold Press and Sandy Petersen Games provide alternative 5e supplements of very high quality
Because of the rules. Yes there are probably games out there, that does some aspects better. But generally and overall the 5e rules are sent from heaven. They are SO easy that I as a DM can manage them at the table, but still complex enough that they provide my players with enough tactical options when playing. Seriously, the 5e rules are just amazing at the table.
Because my players love it. I have on rare occasions tried some different games as one-shots: CoC and WFRP mainly. And while they didn't hate it, the games just doesn't provide that magical mix of plot-driven combat that 5e does. And when we then went back to 5e the next week, they were all just like "aaaaaahhhhh this is gooooood".
Because of the leveling system. My players are not murder-hobos. But they love leveling up. And again 5e provides just the right mix. It doesn't take forever to level up, but they stil get something new the can play with. It's quite amazing to see people at my age being so happy about leveling up a make-believe character, but that is what 5e does for us.
I have yet to find another RPG that gives me all of the above. I am sure we'll try other games on occasions, but we will almost 100% go back to 5e again.