[dchest]

It has to do with defender's vs attacker's costs. PBKDF2, which is usually instantiated with SHA-2, even with huge amount of rounds is still a lot cheaper for the attacker than for the defender, since the attacker can use GPU/ASIC, requiring fewer transistors, running many calculations in parallel, while defenders usually use CPU. On the other hand, bcrypt, scrypt, Argon2 don't provide a lot of advantage to the attacker compared to CPU, since GPU and ASIC implementations are expensive and memory-bound.

PS My measurements show that pure JavaScript implementation of scrypt is better than fast native PBKDF2 provided by WebCrypto API or Node.js at the same running time.

PPS But yeah, if you can't use bcrypt/scrypt/Argon2, but can use PBKDF2 with high number of rounds, sure, do it.