This has been discussed many times over. When running a program like strictly, you need impactful dances for the last bit, not saying that they are the best dances, because they often aren't, just that they are impactful.
Whether we like it or not, Pete has name recognition above most celebs on this years strictly (not saying that he deserves it or not, just stating what I believe to be the case). Plus it looks like he can put a show even if his dancing isn't perfect. Besides he has had three crowd pleasing dances in a row as has Anita.
I believe that Anita's and Pete's treatment is fairly similar so, they both aren't the best dancers but obviously have the performance chops and nerves of steel that suit for big finales.