Several reasons....
As I mentioned, I don't like the ergonomics of a T-Bird, though I love the sound. They aren't comfortable for me to play at all.
Also, I'm not interested in making a "clone" of something. If I wanted a T-Bird i'd simply buy a Gibby/Epi/Lull/Bach/Orville/whatever.
Ok, there's also a third reason..... Because the T-Bird "shape" doesn't lend itself to being appropriate for many gigs. The sound does, but not the shape. I make my living making music, so i always have to be conscious of what bass I bring to a gig...need to make the artist happy...and a T-Bird simply wouldn't get brought to many gigs.
And there's a 4th reason.... I do a lot of fly in gigs, and a T-Bird, due to the size, would always have to be checked baggage...whereas my Fenders, Lakland, Danos, etc are small enough to fit in the overhead bins on Southwest (and JetBlue).