We know that alkenes are unsaturated hydrocarbons where as alkanes are saturated hydrocarbons. Now unsaturated hydrocarbons are more reactive than saturated hydrocarbons. This is because saturated hydrocarbons contain only single bonds which are very stable and difficult to break. On the other hand, unsaturated hydrocarbons contain pi bonds, which can be easily broken as they are more strained. In other words, pi bonds being weaker can be easily broken. Also, the electron cloud of the pi bond is placed above and below the plane of the bonding atom. This results in easy availability of the electrons in the unsaturated compounds, making them more reactive.
So, alkenes are more reactive than alkanes due to the presence of a carbon-carbon pi-bond.