Why don't periodic acids convert alkenes to 1,2 diols?
Sodium periodate can be used indirectly to form diols from alkene, see:
Emmanuvel, T. M. A. Shaikh, A. Sudalai, Org. Lett., 2005, 7, 5071-5074.
The mechanism for periodate cleavage of diols also strongly suggests the periodate is an electrophile.  This may explain why in general, alkenes are not nucleophilic enough to react under normal conditions without a co-reagent. The osmium tetroxide reaction is favoured as a [3+2] cycloaddition mechanism.