In Enterprise, agent Daniels informs Captain Archer that the mysterious figure who aids and directs the Suliban is someone from the 28th century, when they haven't yet discovered how to physically travel through time and can only send back transmissions.
Yet, in the finale of Voyager Janeway uses a 25th century piece of Klingon technology to travel to the past. Was the mystery figure not aware of this 300 year old technology? Wouldn't it have been developed and perfected even more over 300 years?
(As for why he didn't use the slingshot effect, as noted here it is a very unsafe method, and may have even been forgotten, as it was a closely guarded secret and was only discovered accidentally)