It's the same reason you don't see high flying stocks like FB, LNKD, YELP, etc. actually getting buyout or tender offers for their shares.
In the event of having to sell a large block of shares, all of the share prices will crater, Alibaba included. Prices are set at the margins, so until there is a stampede the stock price will go with the flavor of the month.
Stock price != company value. Nobody will enter a position they can't get out of easily unless there is real worth in holding.