k-1

The IRS determines filing dates and deadlines.

We are not given the reasons for the dates they choose. 

You can file an extension if you need more time to file after getting the K-1. 

Actually, the original due date for partnership returns is 3/15 so if you got one in April it was on extension. some don't file their return until close to  9/15. at one time the original due date and extended due date were 4/15 and 10/15, the same as individuals. In those days many individual returns had to be filed using estimates and then amended.