[natep]

Limited budget is certainly a big reason, and another is the lack of a pipeline. When Apple releases a new iPhone every other year, it's not like they started development on the next model only after the current one is released. Development is staggered so that there is always something at each stage of development.

As it says in the article, they will be leveraging proven technologies, but have not yet solicited proposals for instruments. This means they are still at the very beginning of the development process, and more money would only help so much. That's why the planetary.org post says that a higher budget would only move the launch date up 2 years. If they had started on a new rover 6 years ago with adequate funding, then we could be looking at a new rover in a year.