Lord Shiva is the deity of destruction and the dark forces. So, the dark night is naturally dear to him. The night is the time of the predators when they set out to enactnightly destruction of life. The dark forces set out to torment the world in the form of ghosts, spirits and shadow.
The dark nights of the descending moon phase encourage criminal and immoral activities. Shiva is worshiped during this period of darkness to keep him in benevolent mood to keep dark forces in check.
All the 14ths of the descending moon phase of each month are refered to as 'Shivratris' , but this Falguna month shivratri is called the Maha Shivratri, Because it occurs in the vernacular last month of the year. It is the last Shivratri of the year.
There is one more reason. 'Maha Shivratri' comes right after the autumn when trees stand denuded of their leaves. All the leaves have fallen down to become a carpet of seasonal destruction over the earth. The trees stand bare providing a picture of desolateness, the happy hunting ground of the dark forces.