Governor and masquerade both sound like bad ideas when you go witch.
Looks to me like the other guy got a lucky gold T4 (in which case the navigator is really helpful, as it skips half the deck to directly find that gold again on T6).
Then, as Davio said, he just had tempo on his side while you were gearing up for a long game that never came.
The witch and the governors both help the long term game, but really suck short-term (and are anti-synergetic). The fact that he had navigator+cartographer in his deck, and masquerade+governor in yours to mitigate the curses made sure your edge was really only to kick in turn 17+. Which means he had an edge before that.
The fact that the game would end before t17 was unclear, and to be fair, I would venture the odds of that happening at 60-40, in his favor on T1. Obviously on T6, his odds shot up by a lot.
To explain why all options of governor seem bad to me on this board once the opponent has a clear edge in money:
-Making him draw cards is making sure the provinces will run out fast (both you and him will hit 8 pretty consistently: he has more money on his side).
-Giving him silver drowns out the few curses you give him: again, with money/tempo on his side, this would also make him win. He could also switch to a gardens deck, given that you would have an insane number of cards pumping up his deck (witch + governor).
-And the trash option would give him a free way to trash up the curses you give him, essentially turning your witch into a moat and your governor into a remodel+discard attack.
All in all, this is one of those boards where lots of good cards taken together counter each other to form a very average deck. Makes sense that it would have average odds against BM.
It's an interesting board. I would have gone masquerade-BM, personally. I am pretty sure it beats navigator and witch, but unsure if it doesn't get run over by governor (without witch).
And I like navigator as an opener, when aiming for a quick game, or cycling a crucial card (witch/gold here) as often as possible. It's surprisingly good at quickening games.