So that it will not to be a fard
 

Hz. Aisha (RA) narrates:

One night, the Apostle of Allah performed nafila (supererogatory) prayer in the mosque and a few people followed Him. The next night He performed again; this time the congregation increased in number. On the third night, the congregation which increased in number were waiting in the mosque again. However, the prophet Muhammad (PBUH) did not come. The next night, he explained the reason of his absence:

“I saw that you had gathered. However, what prevented me from joining you was my fear that this worshipping would become fard for you”