You should never get a rose comb chick if you have two single comb parents.
Because single comb is the recessive gene, it only appears when the chicken is pure for that gene, and then it breeds true.
I think that is the main reason single comb is so common: once you have a flock of chickens with single combs, no other type will show up.
(Pea comb, V comb, and several other comb types are caused by other genes but are also dominant over single comb, so they do not pop up either if the flock has only single combs to start with.)
If you want to know if a particular chicken carries the not-rose gene, you can test mate that chicken to one with a single comb. If any chicks get a single comb, then you know the rose-combed parent carries the not-rose gene. If you get a dozen or so chicks that all have rose combs, then the parent with a rose comb is most likely pure for the rose comb gene, because they are passing it on to all of their chicks. (Of course you know that every chick produced in the test will carry the not-rose gene, because they have one parent with a single comb. So those chicks all get butchered or sold as mixed-breeds, even when they show a rose comb.)
Test-mating every chicken before using them in a breeding program can take a fair bit of time and effort, which is why hatcheries typically don't. But if you are determined enough to do it once, you can end up with a flock that breeds true for rose comb. A sort-of compromise is to test the rooster each year, because he's just one chicken, and if he is pure for rose comb he will never produce a chick that shows a single comb, but you won't know if the chicks got the not-rose gene from their mothers.
