The main culprit is NSA Suite B -- or, rather, people who read that document as something that it is not. NSA Suite B says that NSA uses curves P-256 and P-384. Some people have chosen to interpret that as "don't use any other curve". P-521 (aka "secp521r1") is not one of these curves, hence its removal.
There is nothing wrong with P-521, except that it is, in practice, useless. Arguably, P-384 is also useless, because the more efficient P-256 curve already provides security that cannot be broken through accumulation of computing power (see this answer for a discussion on that subject). Yet people and even NSA uses P-384, which goes a long way to show that the choice of what elliptic curve to use in a given cryptographic protocol is not fully a matter of pure rational logic and security assessment. People who are in the position to decide whether a given curve is acceptable or not will make choices based on their perception of how their choices will be perceived by other people who themselves are not necessarily the most logical of thinkers (or not at least perceived to be such).