why did force ghost obi wan choose to visit luke right before he was about to pass out?

Obi Wan knew that Han was going to find him in a short time.

There is a school of thought that Luke completely lost it after seeing his Uncle and Auntie (who'd raised him after he was abandoned almost at birth by his parents) burned to a crisp and subsequently seeing his next father figure chopped in half.

He then started hearing voices in his head when right after this horrific event, he was asked to undertake a mission in which failure would have resulted in thousands of deaths and the destruction of an entire planet.

And then, right after barely escaping being eaten by some horrible monster, he's exposed to extreme weather conditions. At this point the "voices" he was hearing escalated into full on visions.

Hardly surprising. The kid had to deal with a lot...