The day is observed because Lord Swaminarayan has stated in the Shikshapatri to do so.

All disciples must also observe the vows of ekadashi (11th day of each half of every month). Such vows must also be observed on the birthdays of Krishna etc. and on Shivratri, and these occasions should be celebrated with great ceremony. ||79||

Shivratri is one of the common festivals observed in India, and the Lord has commanded us to observe it.