As one of the few protestants on this board who switched over from being baptized a Catholic, I can answer this quite succinctly.
The entire protestant denomination came to be to fight the catholics and all their non-Christian teachings. Catholics teach people numerous things which have ZERO references in the bible such as:
1. That saying some set number of prayers forgives you of sin. The bible clearly states only asking the Lord for forgiveness can forgive you of sin. No Earthly actions can do that.
2. They made up a prayer called the "Hail Mary." The problem with it is that it talks about Mary being holy. Mary was not holy. Mary was blessed, but she was not holy.
3. Catholics pray to dead sinners. After relatives die, they pray to the dead. The Lord forbids this as he clearly states that you can pray to the father, son or holy spirit only. Anyone or anything else is a violation of his commandment regarding not taking any other Gods but himself.
4. Catholics have "confession" where they confess their sins to another sinner and so-said that sinner forgives them. The bible clearly states that ONLY God can forgive sin, no Earthly being can do that.
I could go on and on. Catholics are Christians, but their beliefs are so whacky I can see why some would say they aren't Christians at all.