The answer is the manufacturing costs and difficulty in working with ti vs. the benefit vs. steel/aluminum/carbon being fairly slight in a fork. Very few wanted to take on that project because few consumers would want to pay for it. This is a bit flexy on the front end, but not terribly so, and it's a QUICK climber.

They do exist...Panasonic made one with a ti fork...Fuji...

This one is a rather interesting example with ti fork, stem/bars (all one piece) and seat post.