Because they don't care / interested. Why ? I don't know. Even the famed XF had on a vB convertor 3 months back. I haven't checked further. It is possible that XF is aiming forums from vB as a priority for god knows reasons. One that I can think of is that there are many very very big forums that still use vB to continue the legacy backend.
There was not even IPB support. Personally when we see that, free software has much better support for convertors.