There are a couple main reasons, some of them I believe subconscious, for why Catholics are singled out.
- They believe themselves to be exclusively the Church that Christ established, and other Christian sects (with a few exceptions) as adhering to heresies which bar them from ecclesiastical unity with them. This is an example of the Catholic Church drawing a distinction between themselves and other Christian sects.
- The "Protestants" are called Protestants because they formed out of "protest" against perceived corruption in the policies, praxis, and theology of the Roman Catholic Church. This is an example of the Protestant sects drawing a distinction between themselves and the Catholics, although most still would graciously assert that, as Catholics believe Jesus is God, Catholics are saved despite their theology and praxis.
Every main branch of Christianity is in some sense "singled out". It may simply seem that the Catholics are "singled out" because their branch of Christianity is united as a single sect under the Pope, while the Protestants are 40,000+ (or something) fragmented sects of Christianity existing in large numbers in most of the countries where Catholicism has a big presence, but are united only in in the confession that Jesus is God (a union through the least common denominator, choosing to name all other doctrines as secondary in importance when it comes to unity and, in most cases, also salvation).
There are also, however, other branches of Christianity which are also separate, or "singled out".
For instance, the Eastern Orthodox (Calcedonian) Communion, the Oriental Orthodox (non-Calcedonian ) Communion, the Assyrian Church of the East, and the modern-day Arians (Jehova's witnesses, Unitarians, Mormons, etc.).
It's just what happens when there is a major departure in theological understanding in a religion - those who believe one way need some way of differentiating themselves from those that believe another way that is deemed improper or incorrect by some adherents of the same religious umbrella (in this case, Christianity).
At the end of the day, the Catholics single themselves out as the only true Christians because they believe any non - Catholic sects to be in heresy, and therefore, not a 100% true expression of Christianity.