One big factor: traditional brokerage/investment accounts provide records of all stock transactions, including the cost basis and precise gain or loss. This typically comes via the 1099-B. It's straightforward to report and file taxes in this manner (relatively speaking).
Even the more legitimate exchanges in the US (ex: Coinbase) aren't currently providing these types of records. You may be able to export a list of trades, but often cost basis is missing. Or if, for example, you've moved coins through various exchanges, or traded one coin for another coin (say BTC to ETH), before converting back to fiat currency, it's very difficult to figure out how to report everything. There's no 1099-B.
Even the more legitimate exchanges in the US (ex: Coinbase) aren't currently providing these types of records. You may be able to export a list of trades, but often cost basis is missing. Or if, for example, you've moved coins through various exchanges, or traded one coin for another coin (say BTC to ETH), before converting back to fiat currency, it's very difficult to figure out how to report everything. There's no 1099-B.