Because of their double bonds
Why are alkanes so stable? Because the electron cloud distribution is very even.
You have to understand, that everything in chemistry seeks the least energetic configuration and the double bonds in alkenes bear a lot of potential energy.
Why do we have an electrophile reacting?
The double bond creates a partial negative charge, which will consequently attract electrophiles (negative-charge-seeking) reactants.
The double bond will open up and bind to the electrophile. Becuase the alkene has the capacity to bind to more species (due to the double bond), no substitution is required