i think more than anything iowa has had a string of very bad luck with running backs. example: look at coker pace - what, was he at iowa for 2 seasons and is up there at the top of the record books with 18 tds - he stays out of trouble, or whatever, and the RB dynamic is changed.
today - i think a lot of the team and some of the OL, are recovering from the flu from the past week and a long late bus ride.
when weisman came to be, iowa was just in a position of real need at that position. and he filled it. bullock also filled a need - he was a WR and changed positions to help the team.
to answer your question - iowa just had a ton of attrition at that position, for varied reasons.
do i think we'll have a 100 yard back this year? yes.