It think there are a lot of factors at play. Musk wants Twitter* to be an everything app and has mentioned plans to add e2e encryption to the dms. Telegram already had an iffy relationship to people who look for encrypted platforms so he probably wants to set up that idea to get people onto Twitter instead, especially as the right's relationship to the platform is absolutely getting worse.

A big chunk of far right Telegram has moved off the platform recently after quite a few of the groups that got their start from anti-lockdown demos had members get arrested for terror charges. They seem to blame the fact that Telegram is backdoored but they constantly fed-post in open unencrypted channels anyone can read anyway. The police probably do monitor those channels but if I were to guess how they got caught, my bet is honestly that they just put in reports to the cops about each other because being fascists usually goes hand in hand with being horrible people.

I'm not sure how much he actually reads on far right Telegram but there is evidence of him at least following some channels where this sentiment can be seen. If I were to guess he saw oppotunity in bringing that audience back to Twitter.

* I refuse to call it X

EDIT: fix early accidental submission