Kaplan tests are not necessarily harder, they're just designed ever so slightly different from GMATprep tests. I'll give you an example: Kaplan quant tends to focus more on straightforward math, with equations and stuff like that, while GMATprep problems go for more word problems. If you're a recent college grad, then Kaplan math might seem more accessible to you because you've recently had practice with it. I know that was the case for me - I even scored an 800 on a Kaplan practice test!

I'd say that Kaplan tests are appealing to some types of students but not so much for others. You just happen to fall in the former category.