Probably because they'd been talking about this for at least 2 years before he got reelected. Nothing was gonna stand in his way. El Salvador has improved in the short term and that's supported by salvadorans in the country and outside of it. That country had been going through the worst for about half a century between the wars, gangs, murders, poverty and general political instability. He needs another couple years to truly solidify what he's done with the mass lockups. It could've fallen apart with him out of office.
With a country like that, you need a great amount of time and subsequent leaders on your same timing to truly improve it long term. MS-13 was probably the "easiest" issue for him to deal with. Continuing to improve the economy and infrastructure is infinitely more difficult.