why does triod mode sound better than ultralinear?

Not that simple a question - a lot depends on the speaker you are using as well as your listening preferences. Most generally speaking i think most feel that ultralinear is a bit less delicate, for example as ss is to tube, ultra linear would be to triode. Why don't you post your speakers and your preferences - perhaps someone has the same and can comment.

the shorter the signal path the greater the sound. under this theory SET is the best with single- power tube and possibly directly coupled(OTL). there are not too many speakers that are efficient and full range and in cases otherwise the greater power is needed.
i use triode mode in my VTLs despite having low-efficient speakers on the following reasons:
1. i have a small listening distance
2. i can hear more notes and instruments where every part of instrument is so clearly heard that you can hear button switching noise even in big bands.

Despite what you might imply from Marakanetz' post, the signal path will be no shorter in triode mode than it is in ultralinear, though there will be fewer engaged elements inside the power tubes. Unfortunately, with the C-J amps it's not as simple as flipping a 'triode' switch to do this experiment (unlike the VTL's, which BTW are not otherwise wired in ultralinear but regular pentode), as the amp needs to go back to the factory for the modification. Going triode will not be a panacea or unequivocal gain - what you pick up in purity you will probably lose in extension and drive. FWIW, as a MV-55 owner (who now uses VTL's) I once investigated this procedure (since I live about 25 minutes from the C-J factory), and learned that C-J estimates that at least half of the audiophiles who try this mod wind up getting their amp converted back to ultralinear operation in the end. This tells me that unless you have quite efficient speakers, a smaller room, and prefer to listen mostly to smaller-scale acoustic and/or vocal music, you'll probably be better off overall using your MV-60 the way it came.