In June 2017 the IRS requested information on all users of Coinbase ( From Maj 2016 re brand to GDAX ), but the number was reduced by a court order in November.
Now number is around 13000 users.
Users that have invest more than 20.000$ in crypto between years 2013-2015
The exchange must provide information’s within 21 days and must include : ID, name, birth date, address, and transaction records from 2013-2015
If we know that bitcoin price in 2013 was around 100$ and if we assume that 13.000 users have invested around 30.000$ each that is around 4 million bitcoints which will worth this days from 400 to 800 billion dollars.
This is maybe the main reason behind IRS interest in crypto.