Obi-Wan's Death?

[Dori]

Obi-Wan saw Luke Skywalker and his companions entering the Millennium Falcon and decided to allow Darth Vader kill him with a smile on his face. As he died, his body vanished into thin air, leaving only his cloak and lightsaber behind.

He could have still fought on but decided not to, why?

 

[Igba]

I think he wanted to give Luke Skywalker and his companions the opportunity to escape Darth Vader. He let Darth Vader enjoy the moment of killing him so Luke can escape.

 

[Jon]

I think he wanted to give Luke Skywalker and his companions the opportunity to escape Darth Vader. He let Darth Vader enjoy the moment of killing him so Luke can escape.

This is very correct. Yes, it's possible that he could have kept on fighting Darth Vader but it's going to put Luke and his compatriots in more danger. It was why he let Darth Vader get that final blow.