Why don't periodic acids convert alkenes to 1,2 diols? Periodic acids cleave 1,2 diols by forming an periodate ester as an intermediate. Osmium tetroxide also forms an osmate ester with alkenes and yields syn diols after a hydrolysis step. Given that their structures are so similar, why doesn't periodic acid also succeed in forming diols with alkenes? (These in another step will be further cleaved to ketones). Instead in the Lemieux-Johnson oxidation, osmium tetroxide is taken in the first step - followed by periodic acid. 
Osmium tetroxide does not cleave 1,2 diols - though I thought it might be in its being not such a strong oxidising agent as periodate - it may not suffice to cleave a C-C bond. Why doesn't though periodic acid carry out diol formation as well?
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.