Why does canned mango pulp have no vitamin A?
The vitamin loss is due primarily to the puree'ing. The viramin A in mangoes comes from beta-carotene, and beta-carotene breaks down easily due to oxidation. On a similar note, I've seen recommendations that carrots (another good source of beta-carotene) not be chopped into small pieces before canning to preserve their vitamin content.

Lots of vitamins are also destroyed by the heat of canning, but beta-carotene is not as susceptible to this problem because it is fat soluble.