A security failure at some point resulted in a person or persons, as yet unknown, interfering with the flight crew's performance of their duties, for the purpose of terrorism. The crew died trying to thwart that effort, resulting in the tragic loss of the aircraft and all on board. As for why no group has yet claimed responsibility, they failed. Whatever building or major city was their target is still intact. No terror group will advertise their failures.
This plane flies west over the populated land of a relatively sophisticated country. Yet no-one with any recollection of UA 93 makes a call from a concealed phone.
Either the passengers are dead, unconscious or unaware what is happening.