Because heretic is a technical term which means somebody in heresy. Heresy is technical term, as well, and defined by the Church beginning 1500 years before the Reformation.
Romanist and RC were intended to be perjoratives. There is a difference. The term Protestant was coined by the Protestants, as well. Self identification. If the Church referred to itself as Romanist, that is one thing, but the fact is that the term was invented by and came into fashion with British and American anti Catholics.
Origin of the word “Protestant”
May 31, 2008 by Kevin S.
Back in Luther’s time, Germany was ruled by princes, both Lutheran and Roman Catholic. If Charles V, the Roman Emperor, had his way, he would have forced all of Germany, including all of Europe, to remain Roman Catholic. Six of the princes who ruled Germany had already converted to Luther’s view, including its 14 Free Imperial cities. They considered themselves Evangelicals or Lutherans (or followers of Luther).
Charles V gave to order (Edict of Worms) to enforce all the German lands to remain Roman Catholic But thank God those six Lutheran princes courageously stood together, united and strong, and refused to accept this rule. They protested, made their case to demand that Lutheran lands should be free to remain Lutheran, and where the prince was Roman Catholic, that state may remain Catholic.
Then, on April 29, 1529, the six princes declared to Charles V:
“We protest before God and before men that we and our people will not agree to anything in this decree that is contrary to God, to His Holy Word, to our right conscience, and to the salvation of our souls.”
Charles V could do nothing because the princes stood up in unity for religious freedom. From then on, they were known as the Protestors or the Protestants.
ORIGIN OF THE WORD PROTESTANT
Also, your claim that "Romanist and RC" were intended to be perjoratives is also without merit. While it might be meant as a pejorative by some it is meant as shorthand by others.
Frankly, your use of the term "Protestant" is frequently meant as a pejorative while, to others, it is merely descriptive.