I recently submitted my thesis for examination, and it is my 4th draft too. The feedback from your supervisors is based upon what an examiner might ask when he/she reads your thesis. Writing for an audience is difficult, and your supervisors are helping you navigate these treacherous waters. Remember, since you have conducted the field work and read all of the literature, everything you rite makes perfect sense to you but not to an outsider. There's nothing wrong with you or your ability to write. Keep your eye on the prize, which is completion. Good luck!