Because of the Freemason's beliefs and practices.
Freemasonry promotes indifferentism, the heretical (to Christians) belief that all religions are equally legitimate attempts to explain the truth about God but that this truth will never be explained. This is incompatible with Christian faith because Christians believe that God has definitively revealed Himself in the person of Jesus Christ, and desires that all men come to the knowledge of this truth.
Freemasonry's teachings and practices also result in syncretism which is the blending of different religious beliefs into a unified whole. This shown in Masonry's religious rituals, which gather men of all faiths around a common altar, and place all religious writings along side the Bible on the Masonic altar.
The Lodge's practice of requiring its members to swear immoral oaths is also incompatible with Christianity. These oaths require a Christian to swear on the Holy Bible that he will uphold a code of moral conduct that prefers Masons to non-Masons, and to preserve secret passwords and handshakes. Such oaths are immoral because they are trivial and not necessary. These oaths are sworn under penalties of physical torture and death (e.g., having my throat cut across, and my tongue torn out by its roots). These penalties show a lack of respect for God.
For more information, see: http://www.scripturecatholic.com/freemasonfaq.html
Add: http://www.archdiocese.la/about/faq/index.html
With love in Christ.