The Fifth Doctor dealt with a very difficult regeneration sickness, so severe that he needed to sequester himself inside a Zero Room environment which was free from the "noise and distraction" of the universe. Many fans point out that he was quite a distance away from the TARDIS when this regeneration occurred, and since the TARDIS seems to help him with regeneration, perhaps this is why the regeneration sickness was so much more difficult this time around. Or maybe the Fifth Doctor just needs to man up.