As far as I know, XMPP requires keeping a connection alive when you want to receive realtime messages. On most mobile OS, keeping alive that connection and responding to it requires keeping the CPU awake, which severely affects battery life.

TextSecure can use the mobile OS's built in push notifications, which gives you realtime message receipt without killing your battery.

XMPP may be superior when it comes to security/technology but TextSecure is superior when it comes to usability and practicality on mobile devices.