I'm not sure why either, but you are right, this is pretty common. I often hear that people find
Trista and
Tristan to be very attractive if it weren't for that pesky sadness connotation.
People don't seem half as bothered by other ones though... like "Mal", as in
Mallory or
Malcolm or
Malachy. Yet "Mal" comes from the same root as "bad, evil, ill, etc" (those roots are not connected to to "mal" in
Malcolm or
Malachy at all, but neither is the "trist" in
Tristan related at all to the French "triste" so go figure!)
It's very interesting.