The Chofetz Chaim answers that generally Teshuva is not a natural process – since when can we reach such intensity and regret that we are allowed to undo past sins?! Rather, everyone receives special help from HaShem to do Teshuva. Pharoah simply did not receive this help to repent. But his freewill was not completely removed from him; if he wanted to genuinely do Teshuva, he could have. [and actually I have heard that indeed Pharoah did repent in the Red Sea; and said the words ‘me chamocha b’elim HaShem…’ which is why the first chamocha is pronounced with a CHaf sound, but the second is Kamocha; with a Kaf sound – because Pharoah was drowning as he said
this, and saying Kaf allowed him to keep his mouth more closed and thus to finish his sentence without letting water in]
