He probably can't. The lowest paid NFL head coach makes $3.5 million per season, and has other money making opportunities (endorsements, speaking opportunities etc). I doubt teams pay their coordinators that much. And let's not forget that if he has success the salary upside goes much higher for a head coach. Top ones are getting $7-8 million per year, and there is no way to get that as a coordinator.
Aside from the money, the obvious reason to go somewhere else is for the opportunity to be the head guy and run things as he sees fit. If a guy wants to be an NFL head coach he would be barking up the wrong tree passing up opportunities waiting for a highly successful team to open up their head coaching gig just for him - head coaching openings generally only come up with teams that are having problems, and even then there is competition for the job. An assistant coach or coordinator waiting for the perfect payday and perfect team that considers him the perfect guy above all others to fill the job is looking for a needle in a haystack. And remember, guys like Richard are competitors and have ambition, and they are looking to reach the pinnacle.