There's a few reasons:

The vast majority of revenue comes from the console versions, so they take priority. From a managing our business stand point, we aren't going to hold them back to wait for the PC to be ready.

PC requires optimizations for dozens of different possible configurations (the arcade unit, while a PC, has one specific configuration to test and optimize for...).

Compat testing across all of this hardware takes a long time.

Online integration is seperate and different from the console versions, requires different and additional testing. 

Addressing the technical requirement needs for Games for Windows certification takes time, as does submission for approvals for the program (similar to a console submission in time required but less rigorous).

Etc.

In our ideal world of worlds, they would be simultaneous. As it is, we're going to keep them as close together as possible. Exactly what that means in terms of a gap, it's not certain yet.