Some prefer to use HttpClient because it is already built into the framework. So there's no need to add extra bloat to your project.
RestSharp, like any library, is easier to use because someone already did the hard work and ironed out the problems gotten along the way.
I have used both, and the one is not better than the another. It just depends on your use-case, situation and environment.
I had a WebAPI proxy project that needed to call another REST API. I chose RestSharp at first because I was already familiar with it from using it in another project. However, I changed to HttpClient eventually because it uses the same return types as the WebAPI controllers, so it just saved me time from having to convert RestSharp's responses back to a HttpResponseMessage.
Of course each one has its pros and cons, it just boils down to your use-case.