Prataman and Ian are both right.
I'd like to add that money is the main thing like what Prata has said. If you asked any PBA professional, most of them - if not all - would take a PBA title over a larger winner's cheque any day. The PBA pros know that bowling doesn't pay well. Heck, each season is only what, 4 - 5 months? The rest of the year, most of them have their other jobs like gas station attendant, insurance agent... In other words, the money isn't enough. And i'm talking about those good ones who win often. Some have been on tour for 10 years and still have less than US$100,000 in
total earnings.
PBA is for the titles. It is recognised that way. Why?? The reason is in Ian's post.
The PBA is much more gruelling than the scores for each bowler accounts for. They face conditions that are far more difficult than regular USA houses and leagues (in the states, many league and recreational bowlers average above 220 - 240). Let alone the rest of the world... Many amatuers cry foul when they play in the PBA and give up, preferring the ego boost that amatuer tourneys offer.
All this may not mean that Mack isn't good enough for the PBA. One thing is for sure: he won't be as good as he is in amatuer tourneys. Robert Smith and he travelled together. I've seen them both several times in Singapore. I remembered reading about it when Smith decided to go Pro...
Even as amatuers, Smith was the better player. But playing in the PBA has made him even better. He has only two titles (i think) so far, but if you know how he played before he turned pro, you'll be able to see his improvement.