Why can't more than one sperm enter an egg?

Only one sperm normally fuses with an egg because if more than one sperm fuses, through a process called polyspermy, development of the zygote usually stops.

Fertilization of one egg by two sperm is possible, but the resulting zygote is almost never viable because such a zygote would have three sets of chromosomes instead of the normal two. This condition, called triploidy, is usually incompatible with life. Those rare triploid infants that do survive to term have severe and multiple birth defects and rarely live more than a few days.