Why is it in some traditions, mainly the Theravada traditions meditate with eyes closed and most mahayana traditions meditate with eyes open? Was this just a naturally evolving thing as Buddhism went along?
I myself am practicing (Or attempting to practice) Zen which is quite strict on eyes open. When I do it at home I must admit I do sneakily close my eyes sometimes cos I like it. It got quite bad for a time that I even considered moving traditions, but I can't, cos Theravada isn't my cup of tea, neither is Tibetan Buddhism, though I have respect for them nonetheless.
Therevada seems to use the arising, abiding and passing away of sensations as the primary method of vispassana.
Zen teaching says generally that people keeps eyes open in order not to fall asleep but also to keep too much ki from building up as a result of meditating on the hara (although I don't remember the last time I heard this explanation).
I haven't heard non-Zen Mahayana address this. Vajrayana gets their instructions from Indian sources and basically just follows them however those basic instructions are not really correct (I don't remember where these basic meditation instructions come from - they have been read to me at least twice and I've read the source previously too - pretty much Tibetan sources insist that you shouldn't blink).
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche