Hmm.. so you put the mayo on the meat, then stick the knife back in the jar and get meat juice in the jar. So now the meat juice rots in the mayo jar, and that's not very appetizing.
I'll stick to putting it on the bread, as a few possible bread crumbs in the mayo is not such a big deal.
Plus, it's just a lot easier to spread mayo on the bread. Not to mention the fact that in all actuality it doesn't matter since once you put the sandwich together, the condiment spreads to the meat/chees/tomato/other stuff anyway.
Unless you like your mayo on the outside of the sandwich?

Nearly all men can stand adversity, but if you want
to test a man's character, give him power.
--Abraham Lincoln
Excerpt from Animal Songs Never Written
"Hey, dad," croaked the vulture, "what are you eating?"
"Carrion, my wayward son."
"Will there be pieces when you are done?"