The jump from college to proball is, if anything, steeper than the jump from HS to college. Add in other unique factors and suddenly the game gets hard, really hard.
A player gets to college baseball by capitalizing on his strengths and papering over his weaknesses. A position player earns college playing time primarily with his bat (everyone who hits will play). So, one will frequently see (e.g.) a rightfielder who can hardly throw to second base, but is rolled out to play every game because he rips. While ripping is still king in proball, there is only one DH per game and a player cannot paper over or hide skill weakness. For example, a college catcher may only face stolen base threats from 3 guys per college line up; at the pro level, it will be 5 - 6 guys AND each is faster then a college stealer. The pro game is, quite simply, faster, harder and unforgiving (all a college guy needs to be is better then a recruited walkon; a proplayer needs to be better then the latin pipeline, draftees, and an unlimited number of players the club can sign).
The first unique thing a player heading to camp after the draft sees are Latin players. About a third of each roster is made up of players who have been playing professionally since they were 16. That means 360 days a year for 4 - 5 years; playing with wood, competing just to get to US camp. Only the best of the Latin best get to US camp - and they are good, boisterous, and already understand the pro grind. These players are hungry, incredibly hungry.
The second thing they notice is the sheer number of hours devoted each day - seven days a week, 30 days a month - to the game. While a league like the Cape beings together the cream of US college players, there are 5 games a week, the season is 10 weeks long, most games are show and go, there are days off, limited travel, etc. The only real similarity to proball and the Cape are wood bats.
Third is the lack of $$$ - to eat, drink and be merry. Seven games a week, most being at night which means the player "caffeine ups" for a 7:30 game and "beers down" at midnight - seven days a week. A totally different regime is proball.
Fourth is the lack of any support system. No real friends (your friends are actually trying to get you fired), no ability to decompress after a bad outing (each player constantly ruminates over his current status), no girl friend to sooth you, constant barrages of club stats proving you need lots of work and questioning how you even got to proball, sleeping weekly in strange, extremely low end motels, eating (on the road) crap food (the power conferences make sure their players have good nutrition; you're on your own in proball).
Fifth, the pressure is different. Until a player adjusts to proball rhythm, he is "paranoid" more than college. And, btw, he's right. So, a routine slump builds - especially if the player has never slumped before.
Sixth, the pro players are better. A college player can hit 300 by teeing off on mid-week pitching, or college relievers, or seeing a good pitcher for the third time. In SS or even low A, a hitter will not see a pitcher for that third time - pitchers go maybe 5 - 6 innings because they are pitch count limited by the club. And the relievers who then enter were most likely SPs in college - so no free lunches - ever.
Seventh - and this is after the first pro year - is the amount of effort needed to advance - and the effort MUST being results. So, that weak throwing outfielder had better improve his arm strength rapidly in the off season (while continuing to improve his hitting skills). That takes time and money - way more time then even the top college programs and way more money.
Finally eighth, it may only be a routine slump.
I could go on and on; but the point is past results don't guarantee future returns. Proball is hard, really, really hard.