One reason could be because he was "martyred":
he steadfastly refused to take the oath of supremacy of the Crown in the relationship between the Kingdom and the Church in England. Holding fast to the ancient teaching of Papal supremacy, More refused to take the oath and furthermore publicly refused to uphold Henry's annulment from Catherine. John Fisher, Bishop of Rochester, refused the oath along with More. The oath reads:
...By reason whereof the Bishop of Rome and See Apostolic, contrary to the great and inviolable grants of jurisdictions given by God immediately to emperors, kings and princes in succession to their heirs, hath presumed in times past to invest who should please them to inherit in other men's kingdoms and dominions, which thing we your most humble subjects, both spiritual and temporal, do most abhor and detest;
Because of this, he was then charged with high treason, and was convicted, and consequently, decapitated. I suppose, the Catholics believed this to be "martyrdom" as More stood for his beliefs rather than compromise, even on pain of death, and so, they canonized him. That, is a possible reason.
Edit: This is not the sole reason he was canonized. This, can be combined with the fact that he had great reputation:
The steadfastness and courage with which More held on to his religious convictions in the face of ruin and death and the dignity with which he conducted himself during his imprisonment, trial, and execution, contributed much to More's posthumous reputation, particularly among Catholics.