The main reason why it is used so much is due to its ability gale wings, which allows Talonflame to use massively powerful priority Brave Birds, which will leave a dent in almost anything that doesn't resist it, especially when equipped with a choice band.
Another thing is that it has decently good coverage with its two stab moves, Brave Bird and Flare Blitz, which are unresisted by almost everything used in OU, except Tyranitar and Rotom-wash. It is also a good scout through the use of u-turn, and has pretty good survivability through the use of priority Roost, and can sweep through the use of Swords Dance. Talonflame can also increase its bulk as it can invest in HP EVs instead of speed and can run an Adamant nature to increase its power even more due to Gale Wings, as it will already go first because of priority. Even though it has flaws, such as pretty bad stats except for speed, a huge stealth rock weakness and other things, its advantages outweigh its weaknesses.