I am sure very few people posting here can attest to this from personal experience but like above stated, I suspect the volume of stuff you have to deal with as the coach at the collegiate level is far more than the more focused stuff you do as a pro coach. College athletes can be mercurial as they are typically younger living away from home for the first time and conceivably being challenged academically and socially while being thrown into the deepest competition they've yet experienced. They almost certainly need more support from their coaches for everything from nutrition advice to basic life skills. I think it can be supremely rewarding at the end of the day if you are able to see your role defined in a more holistic manner, ie. helping kids transition to fully fledged adults who have learned to translate their passion into self-improvement, but that comes at the cost of having your life subsumed by the 'job'.
While I am sure pro coaches are just as invested in their athletes, the reality is these athletes should almost certainly have more basic life skills, better access to support (mental, nutritional, financial, etc), and have a much narrower focus in their life. You also can choose who you coach and vice-versa. So, in theory, they are there by their own free will and are invested in your coaching. A lot of college kids start off having full belief in their coaches lose it rapidly when they face setbacks or they don't improve as quickly as they think they should. Clearly, this hesitation makes the coaching dynamic challenging because you are trying to convince an increasingly stubborn individual to believe that your process will get results.
Moreover, in the article Mike Smith clearly states that "I’m constantly have meetings in my office, encouraging people to step toward the harder thing. And I think I could have stayed in college coaching a long time, and that’s for sure the easiest option for me. We’ve [had] a lot of success. I feel really comfortable in my position, and this is a whole new challenge for me, so trying to kind of take my own medicine as a coach of stepping toward what’s harder."
Which means he had become really proficient/efficient as a college coach and could ride it out for a long time but he needed a change to challenge himself. This doesn't mean the college coaching is easier than pro coaching it means that he wanted a change up to grow as a coach/person rather than sticking to a tough but familiar pace.