Why did Saint Andrew become a saint?

Saint Andrew was made a saint for several reasons. Firstly he was, according to tradition martyred for the faith (This is an automatic sainthood). He was crucified on a cross shaped like an X. It is from this that we get the term "Cross of Saint Andrew." Which on a side note is the emblem on the flag of Scotland. The second reason he would have been remembered as Saintly is because he was close to Christ, a Faithful Apostle.
St. Andrew was not canonized. He was proclaimed a saint by early

Christians because of his martyrdom. The canonization process did not

come into existence until the 12th century. In the primitive Church,

martyrs were immediately recognized as witnessing to the perfection of

Christian life on earth, having shown the ultimate proof of their love for

Christ by the offering of their lives. By the sacrifice of their lives

for Christ, they attained Heaven in eternal glory and were indissolubly

united to the Lord, the Head of the Mystical Body.