The reason Many Christians don't call Catholics "Christian" is because the Catholic Curch started in Rome, some 300 years after the first Christians appeared, at the time of Constantine who adopted Christianity as Romes state religion, where it became corrupted.
These Catholics introduced a mixture of pagan and Christian beliefs into their new religion. Some of these things were the introduction of the pagan holy day Sunday as the new day of worship instead of Saturday.
And the worship and praying to Mary and saints.
All of these things have roots in the pagan world.
Catholic priest also claimed to be able forgive other people's sins and actually charged fees for the "service". They also forbid the people to read the Bible and only the Priests were allowed to have a copy of the Bible. (Very few priests had copies of the Bible and even less had read it!) Regular people were killed as heretics for reading the Bible in those days.
Catholics also claim that their Pope is Christ's representative on Earth and that therefore the Catholic Church is the only authoritive body on Earth. For this reason the Catholics slaughtered an estimated 50 million innocent men women and children during the Inquisition. Many wre horribly tortured or buned alive, some were burned so slowly that it could be said that they were slowly cooked to death.
Fortunately in the 16th century during "The Reformation" people began to read the Bible for themselves and began to break away from this evil church.
These reasons and more are why Catholics are not considered true Christians by many. After all, who, after knowing all these things about the Catholics, would anyone want to stay in that religion?
Even so, there are many good poeple who are Catholics. I just don't know why they remain in such a church!