All those F-35B models that the Marines are buying and have declared IOC with are LRIP models. There are still a lot of issues to figure out with the F-35 (all variants), and all those LRIPs will have to be modified at a very significant cost. In the meantime, the Navy has Super Hornets to more or less fill the gap until the F-35C is truly ready.
For comparison purposed, the Navy has a fair number of LRIP Super Hornets that saw little use in the Fleet due to their quirks. Even with 25+ years of F/A-18 experience, the LRIP Super Hornets were never really Fleet suitable. LRIP F-35s have even more issues. The Navy is taking the approach of holding off buying F-35s until more issues are sorted out. The Marines need F-35s now so that Marine strike fighter aviation continues to exist.