Why cannot HCN react with ethene?
The &quot;classical&quot; way

Direct addition of $\ce{H-X}$ to alkenes proceeds via the addition of a proton to the $\ce{C=C}$ double bond in the initial step, i.e via a carbocation. This works nicely for strong mineral acids, but not for the weak acid $\ce{HCN}$ with the rather weak nucleophile $\ce{CN-}$.
Synchronous addition?
Q: Can't $\ce{H-CN}$ add synchronously add to $\ce{C=C}$?
A: NO! Such a reaction would involve the interaction of the HOMO (highest occupied molecular orbital) of one reaction partner with the LUMO (lowest unoccupied molecular orbital) of the other. The LUMO of an alkene has a nodal plane orthogonal to the $\ce{C-C}$ bond, while there is - obviously - no nodal plane but a $\sigma$ bond between hydrogen and carbon in $\ce{HCN}$.
The synchronous addition of $\ce{H-CN}$ to $\ce{C=C}$ is symmetry-forbidden!
Catalysis?
Yes! Since the first catalytic hydrocyanation using $\ce{Co2(CO)8}$ (see Philipp's excellent comment above), other catalysts have been developed, namely $\ce{Ni(0)}$ based species using a trypticene-derived chelating phosphine ligand (see image).