Well in an interview with the Fujifilm Blog, Takashi Ueno from Fujifilm explained why the company still does not have a full frame camera as part of its lineup just yet. According to Ueno, he admits that a full frame camera would result in much better image quality, however there are several trade-offs to be made, none of which Fujifilm felt comfortable doing.
Ueno was quoted as saying, “Higher image quality can be achieved with full frame sensors, but in order to maximize the use of the sensor size, the lens will be very bulky and heavy.” He also adds, “The combination will maximize the potential of the full frame, but if you have to carry the bulky lens everywhere to achieve the high image quality, then this is not what FUJIFILM is aiming for.”
Ultimately it seems that the company was aiming for portability and it seems that they have struck a pretty good balance, especially when you consider the image quality that cameras such as the X-T1 or X100T are capable of producing, although it seems that the recently announced Leica Q could give them a run for their money as far as portability and full frame cameras are concerned.