From my years behind the hiring line...this is what probably happened:

While you did OK, you did not ace the 3-1 hour technical interviews, or they thought that you didn't make a strong enough impression in <insert important area for the job/company> - the follow up interview was probably intended to gauge those areas, and you failed to meet their standard.

Don't feel too bad about it, people flunk out of interviews all the time - even if they would be perfect for the job - most places now adopt quite a strict hiring policy because a bad hire is worse for the business than a no hire.

You could try and get feedback form the company but I wouldn't hold your breath. Simply try again with another.