Why does a viper reproduce by giving birth while a cobra reproduces by laying eggs even though they belong to the same family?

Vipers and cobras belong to different families. Vipers belong to the family Viperidae while cobras belong to the family Elapidae. 
Members of Viperidae are generally viviparous (Exception- Bushmaster snake) while those belonging to Elapidae are egg-laying.
Viviparity is an advanced feature in vipers which help them to successfully colonize cold-temperate habitats.  

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