May can have multiple votes until stopped because she's just following EU rules, that voters keep voting until the authorities like the answer.
We aren't having a second referendum because the decision to leave has yet to be achieved, so we have had no time to see how it settles in, and so is way way way way way too soon to be seen as weak in the eyes of the rest of the world, trying to be indecisive and flip flop, simply because some of those who lost the referendum won't accept it.