Why Isha?
Ending the night the way the day began — with Allah.
Isha closes the day. Praying it in congregation, or on time at home, is described in the narrations as carrying great reward precisely because the night invites sleep and ease.
Authentic (sahih) hadith
"Whoever prays Isha in congregation, it is as if he prayed half the night; and whoever prays Fajr in congregation, it is as if he prayed the whole night."
Sahih Muslim 656