ben knew he was beaten with vader in front of him and the stormtroopers to the side. obi wan told vader that if he got struck down he would become even more powerful. ben did not want to become one with the force but he know if he got struck down he would be a force ghost and could help luke even more by being by his side at all times.

of course if this had been the prequels ben would had just done 10 flips across the room and spinned his saber aropund 5 times striking down all the stromtroopers on his way to the falcon.