What are the reasons for not acting upon knowledge despite knowing its merit, such as not praying despite knowing its obligation, and what is the remedy for that?

The reason for committing sins despite knowing their ugliness is the instability of knowledge in the heart. It is not beneficial knowledge unless it is followed by action. Therefore, the Salaf (early Muslims) called everyone who disobeyed Allah "ignorant."

The greatest things that lead to this ignorance are following whims and desires, indulging in lusts, following the footsteps of Satan, and inclining towards the worldly life.

The way to be saved from this is to seek Allah's help and strive against the self to attain beneficial knowledge. This is done by diligently contemplating Allah's names and attributes, Paradise and Hell, and by constantly being mindful of Allah's watchful eye over His servant.

The crux of the matter lies in desiring Allah and seeking His countenance, or desiring Paradise, or fearing Hell. All of this cannot be achieved except by opposing one's whims and desires.