The 8 RB's taken makes sense. I mean, everybody is hoping for this year's Morris or Martin, a guy laying in the weeds who busts out and can be a legitimate RB1. There seems to be a couple of those guys every season who, while not always the top rookie back off the board like TRich, light it up in their first season and become an instant RB1 or even franchise back.
It seems the WR position is more of an investment in that there aren't to many rookie WR's who step into the league as #1 targets for their team plus they tend to hit the sophomore slump too so you could be waiting until year three before they really bust out. Even the best rookies last year (Blackmon, Wright and Hilton maybe...) had around 800 yards and 5 TDs which sets them as middle of the road WR3s. Even guys like Megatron need a year to get going, his rookie stat line is unimpressive, although AJ Green and Julio both closed in on 1000 yards and had 7 and 8 TD respectively.
TE's... I'm not really sure what to make of the position in terms of rookies. Graham and Gronk made an immediate impact (Gronk especially), but even a guy like Gates took a year to get into the system.
Combine these things and the ceiling for a rookie running back is just way higher. If you are the lucky one who drafts the next Alfred Morris, you're set for the next couple years! It is like playing the lottery and let's face facts, fantasy football is all about gambling on sports! We just do it in a way where we can hold it over our friends and coworkers if we luck out and win big!