Because of Nightmare Moon eclipsing Princess Celestia's sun.

The pure negative alicorn magic was released into the world of Equestria and gathered into one spot from which Eclipse was born.

His birth was purely spontaneous and as such was unexpected for the travelling Star Swirl the Bearded who found the abandoned colt.

"What an odd colt. Those are some large red eyes you have there. You shall be known hence forth as Redeyes Eclipse," thought Star Swirl.

The confused and bewildered Star Swirl the Bearded decided it was his responsibility to find the lost colt's parents.