Why was Thomas More canonized? Thomas More was canonized in 1935 by the Catholic Church. During his chancellorship between 1525-1529 in the campaign against the reformation people were killed and tortured in his name, for example, for the crime of owning an English Bible, something that is quite normal nowadays. 

For what reasons do the Catholic Church justify his canonization?
The closest I can find to an answer to this question is in the final paragraph of this article, which says:


  More was beatified by Pope Leo XIII in 1886, along with other English martyrs, and canonized in 1935.  Had he never met death for the faith he still would have been a candidate for canonization as a confessor.  From first to last his life was singularly pure, lived in the spirit of his own prayer:  "Give me, good Lord, a longing to be with Thee; not for the avoiding of the calamities of this wicked world, nor so much for the avoiding the pains of purgatory, nor the pains of Hell neither, nor so much for the attaining of the joys of Heaven in respect of mine own commodity, as even for a very love of Thee." 


(Emphasis mine.)

The implication here is that he was canonized because of his martyrdom, but that even if he had not been a martyr, he may still have been canonized.

This source also supports the theory that he was canonized for his martyrdom:


  St. Thomas More ... [was] canonized for his unfailing devotion.