Just to add a point neglected in the other answers.
Though Catholics clearly acknowledge that they are Christian (i.e., believing that Jesus is God), some Catholics might exclude themselves from being grouped with "Christianity" in a general sense due more to social reasoning vs. strictly theological purposes. Of course Catholics are Christians, but in a social sense, many Catholics do not want to be associated with the word "Christianity" because of the negative connotation that the word "Christianity" has developed, especially in the United States. As a result, many Catholics, especially in the United States, will often separate themselves from other Christian sects in order to avoid gaining a shared reputation of being illogical or backwards.
Many Catholics who study the likes of Thomas Aquinas, Augustine, Irenaeus and other apologists have developed their faith using a very scholastic approach guided by logical reasoning. The long history and list of storied apologists that have come and gone in the Catholic church provide a rich history to support their beliefs. Further, almost all modern science and education have their roots in the Catholic faith and their scholasticism. I would posit that many of these Catholics believe that their faith, then, is reasonable, defensible, and consistent. They likely try to "separate" themselves from other sects of Christianity to avoid "tarnishing" their faith's reputation and to avoid appearing less reasonable, objective or consistent.
In contrast to Catholicism, many (not all) fundamentalist sects of Christianity are considered to be illogical, backwards, and confused, especially in the United States. This is due primarily to their positions on science (e.g., age of world, evolution, etc.) as well as inconsistent approaches to their own fundamental beliefs (e.g., they are "pro-life" but only support protecting the lives of unborn fetuses and often not those of poor persons, prisoners or that of the environment). To be honest, I would suggest that many of the Christian sects in the U.S. have traded true faith and religion for the blinded following of a conservative political party or heretical false prophets. Some of these sects of Christianity are created solely upon heresies or, worse, on the greedy motives of "church" founders -- look no further than the number of TV and radio pastors that take people's money to buy lavish gifts for themselves in exchange for the saving and healing of donor's souls. (Interestingly, these confused sects are returning to the practices that Luther so fervently opposed 500 years ago!).
Overall, these issues have created a bit of a "bad reputation" for Christianity in the United States. The issue, of course, is not all of Christianity, but nevertheless, the word "Christian" in the U.S. often is associated with backwards thinking and a strict political ideology because of the beliefs and practices of many of the 100,000s sects that exist.
Catholics don't believe that the world's history is confined by the text of the Bible like numerous fundamentalist sects of Christianity. The Catholic Catechism as well as other texts (most recently Pope Francis's Laudato Si' and other sermons) also demonstrate that the Catholic church is (perhaps not in practice but in belief) very consistent in it's faith (e.g., "pro-life" means pro everyone's and everything's life). The Catholic church is not perfect, but many of it's Church believe that the church's dogma and catechism are nevertheless reasonable. Though many people despise Catholicism, it's often for the Church's rigidity, "priest problem" or unscrupulous history, not for a failure in their reasoning.