Why do more substituted alkenes undergo epoxidation, but the less substituted alkenes undergo hydrogenation?
As you already know, the alkene has to approach the catalyst surface in the first step. For optimum adsorption, the molecule must be able to orient itself parallel to the surface of the metal. Less number of substituents promote this.   


  The activated hydrogen is in the form of $\ce{Pt-H}$ or $\ce{Pd-H}$ bonds on the surface of metal particles, and hindered alkenes can't approach the $\ce{M-H}$ bonds easily.


Source

Epoxidation on the other hand is free from any such need for to attach to the surface. So we simply consider nucleophilicity.