You need to separate cadence from power output. With the same power, you will go the same speed whatever your cadence (just change gear to match). Cadence is mainly to improve efficiency by spreading the load between legs and heart/lungs.
With a given/fixed power output, you increase cadence by gearing down.
Reasons for low cadence include:
Legs too long!
Wrong kind of muscle.
Cranks too long for your legs.
Saddle too high.

Everyone has a different optimum cadence but it pays to play around and find your sweet spot.