I believe at the time his Jackson were stolen he tried to get Jackson to make him more(he wasn't endorsed), but they couldn't have them ready for a year or something due to Fender buying out Jackson. So ESP contacted him and made him his first signature guitars in a much shorter time frame, so that is why he ended up with ESP. Why he doesn't go back to Jackson is anyones guess. Maybe he signed a deal to stay with ESP for so many years so he can't switch brands, or maybe Jackson is selling enough guitars without having to pay Alexi to play them, so they have no need to contact him to make a signature model.