This is something I have seen become bigger over the past 10 years. It seems since Pujols and Ichiro came around, prospects have become the fad. Then whatever prospect you grabbed, doesn't quite pan out out in his first year and he becomes an after thought. I posted a column last year about this topic from a post about someone keeping Heyward over Price. Heyward had a good year but Price was the better player in the end and it was all about how far Price had fallen because of one disappointing year.
I think a lot of it comes down to people wanting to be the guy who grabs a no name player that becomes a star. It makes you feel like you knew something no one else did even though most people know about Longoria and Braun.
Pujols - 37 hr's, 112 runs, 130 RBI, 1.013 OPS
Hanley - 17 hr, 51 SB, .833 ops,
Tulo - 24 hr, .838 OPS,
Longoria - 122 games, 27 hr's, 85 rbi, .874 OPS
Wright - 69 games, 14 hr's, 6 sb, .857 ops
Votto - 151 games. 24 hr, 7 sb, .874 ops
Ryan Braun - 113 games, 34 hr, 15 sb, 1.004 ops
Prince Fielder - 28 hrs, .830 ops
Ryan Howard - 88 games, 22 hrs, 63 rbi, .923 ops
josh Hamilton - 90 games, 19 hr's, .922 ops
Looking over the names you mentioned, a lot of them had down years between their rookie year and a breakout year where they would have been thought of as keepers in leagues with 5 or less keepers.
Votto - 2009 was a nice year but 25 HR, 80 RBI/R wouldn't have been kept unless you were a deeper league. 1B was so deep that he would have been available in a lot of keeper drafts.
Tulo - 2008 was a bad year. 8 HR in a 101 games wouldn't be a keeper. Not even a question here.
Fielder - Another 28 HR with 80 RBI/R guy that probably wouldn't be kept.
Hamilton - 2009 10 HR and 54 RBI is not a keeper. Injury history and just looked bad. This guy was going later in drafts with few people expecting him to regain his old form.
So just because they had a good rookie year doesn't mean that it will translate immediately.