Board Thread:Questions and Answers/@comment-34921698-20190301142122/@comment-34974603-20190402043507

There's a theory going around that it was because Sans was going to be an extremely hard boss, so Mettaton was turned into an easy boss to save on healing items and lower the game's overall difficulty. Either that or Mettaton NEO was canonically programmed with an entirely different flaw than his EX counterpart. Instead of draining battery life quickly like EX, his NEO form has horrible defense. Considering your power at that time, if he changed into his EX form instead, his boss battle would have still only taken about a few minutes, purely due to the difference in stats.