Why the different release dates for the pc and console versions?

Because they want to focus on getting the PC version out and in good shape before moving on to consoles that have different hardware and control requirements.

Maybe because the pc came out before PS4 and Xbox1.

The console versions were announced almost a year after TESO was announced for PC and Mac. Besides, like Nothv said, ZOS wants to get the PC game rolling before shipping it over to the consoles.

Actually ESO was going to be PC release only. Once Zenimax received the specs for the new gen. consoles then the decision was made to add consoles. This is also why ESO will not and cannot be played on the Xbox 360 or the PS3.

Don't forget; it is a lot easier to patch pc than either of the consoles.

Get as many bugs fixed with the pc version, apply all the patches needed, and then you have the game in better shape to release on the consoles. Microsoft and Sony don't have to put pc patches through certification. Patching the consoles is more involved than just releasing one.