It's economies of scale, basically, when you're debating whether you'd rather have a shiny QB or a shiny HC.
Hiring the right coach is going to allow him to build up a system, thus minimizing risk more than if you're going to simply count on a QB to have dynamite years year in and year out, as well as staying healthy. In the "I want a QB before anything" scenario, you're placing a considerable amount of risk on the gamble that the QB you're looking at may or may not be able to lead your team to victory. Hiring a coach that builds a good system will allow you to stay competitive, sell tickets and merch, etc., in the search for a QB to run said offense.
Don Shula built a system for Bob Griese to thrive in, he changed it to allow David Woodley and Don Strock to succeed, then he changed it to allow Dan Marino to be the centerpiece of the team
Bill Walsh built a system. In said system, it allowed Joe Montana and Steve Young to both win SB's.
Bill Belichick built a system that allowed Tom Brady and Matt Cassel to be successful
The coach has to be able to put his talent in the right position to succeed. If you've got a bunch of talented players and a bumbling idiot for a HC, then the chances are that you're not going to have a very successful year. Of course, there are occasions where you win in spite of coaching, but that doesn't happen all that often.
When the HC fails, it's a bigger systematic failure than if a QB fails. If the coach fails, then you have to scrap the entire thing you've been building for and start completely anew, hindering your progress for a few years (on most occasions). When the QB simply sucks, you just get rid of him and throw someone or something new in until it sticks.
Coaching is more important, IMO, mostly because of the level of risk involved.