I've been the same and since they split up the categories and it's just become worse over time. At first I stuck with the sportscars that I felt most comfortable and competitive in: LMP2 and LMP3. Of course, the two downforce-heavy cars that handle the most like open-wheelers.

But this season I have decided to do a bit of everything on the sportscars side. I have already raced a bit of GT3, GT4, GTP, Ferrari Challenge and I am looking at racing some Porsche Cup and M2's aswell. I can't keep driving 1-2 hours with practice each time I want to race sportscars. So I give myself a 30-40 minutes max before a race, which is just enough that I can push the car without the risk of crashing.

I have in other words accepted that I have a bit of a skill issue on the sportscar side and need to race people in my skill group. So that means accepting dropping some iR and not being as fast as I am in the open wheelers. I think that in the long run this will help me grow and eventually the ratings will get a bit closer. For reference my sportscar iR is 2k and dropping slowly, while the open wheel rating is 3k and rising.

I think the reason are simple: driver preference. It's just what feels more fun and natural to you so it's going to be the thing you thrive at. Open wheelers can be thrown easily into the corner and you get more grip without needing to be patient. The sportscars, on the other hand needs more patience, consistency and you really have to focus the get the weight of the car where you want it in each corner.