Actually that happens a lot. The reason is many applicants see RC as a backup for ESF foundation schools (i.e. KJS, SJS, BHS etc), and if they receive offers from both they will give up RC. So it is typical for RC to hold many more interviews after February for applicants on the waiting list. In fact it is this phenomenon that prompted the new "One Form" scheme, because RC is just tired of being everyone's favourite backup and having to hold more interviews than necessary. The new scheme was going to be introduced this year but has been postponed, so the current situation remains the same as previous years.
To cut a long story short, if your child is not invited to the first round of interviews in January, there is a fair chance that he/she will be invited sometime in March or later, but I would advise you not to totally rely on this and actively devise a Plan B if there isn't one already.
I’m just wondering why most people take RC as a backup ...
There used to be a major reason up to 2 years ago, which was the fact that ESF foundation schools used to receive a subvention from the government while RC do not, so the fees for the foundation schools were about 20% cheaper than RC. This subvention is now removed so there is no longer a significant difference in terms of school fees.
Another reason might be because RC has a shorter history and track record than the foundation schools, so the brand name effect is not as strong.
But by that time most applicants would have accepted a place in other IS and ...
That is not a worry for RC, they will have no problem filling all their seats before school starts because there are always far more applicants than there are seats. It is just the hassle of having to deal with a significant number of applicants declining their offers.