Money, and you have a better chance of reaching top-ten status (& experience) while still in your physical prime. I mean, DUH ......
All depends on the individual, there are arguments for and against for turning professional early. Physical and mental development have to be taken in to consideration, we are seeing a lot more fighters turning professional later and being fast tracked, I believe that's because they are physically and mentally mature enough to do so and also have spent years in the gym sparring professionals and gaining valuable experience. If you're talking level of opposition then yes, normally when a fighter turns over they are usually fighting opposition who are technically far inferior to what they have been facing as a top level amateur but its about the experience they are gaining in those early professional fights but in my opinion they should be experiencing at least something a little physical in those fights. But I think those fights your talking about (the poor, quick blow outs) are more down to poor match making or a need/want for a young fighter to have near 100% KO ratio, just quick or short term thinking.