Basically because the information is transmitted in the form of a
photon, which can't travel faster than light. In the original IBM
proposal you took two correlated photons, put them in boxes, and
carried them to the source and destination of the transfer. At the
source threw in the particle you wanted to transfer, measured the
resulting state, sent that by radio, email, or carrier pigeon, then
cooerced the other particle and photon into the appropriate state.
Since the two photons were correlated, the second particle was then
in the same state as the first.
(I've forgotten some details, but this is the basic idea).
Mark