Because the two sides do not recognize each other, mainland China would never agree to call him "President Ma," and he can't be called "chairman" because he is no longer party chairman of the Kuomintang.
在兩岸互不承認的現況下，中國不可能接受「馬英九總統」的稱呼，而馬總統又不具有國民黨主席的政黨身份。