Because with traditional custodial assets there is a system in which there are verifiable records of ownership from which cost basis can be extracted, and a method for porting this information from custodian to custodian when assets are moved between brokerages. None of that exists for bitcoin, rather by design, as bitcoin resembles more of a bearer instrument than a traditional asset. Except in the simplest case where you buy coins from a single exchange, leave them on that exchange for the entire time you hold them (against all best practices), and then sell on that same exchange, there is absolutely no way for the exchange to know what your cost basis was, whether it was short term or long term capital gains, etc.