Evolution of Live birth (Why do mammal not lay eggs?)

Mammals, like almost all other types of animal today, used to lay eggs, but then they adapted to give birth to live young. why did some mammals make this change and if it has been so beneficial why haven't more animals evolved to do it?

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Sources:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5033709/
https://pubmed.ncbi.nlm.nih.gov/18985616/
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0060063
https://www.researchgate.net/publication/281040772_The_postcranial_skeleton_of_Kayentatherium_wellesi_from_the_Lower_Jurassic_Kayenta_Formation_of_Arizona_and_the_phylogenetic_significance_of_postcranial_features_in_tritylodontid_cynodonts
https://ecoevocommunity.nature.com/posts/38243-an-early-jurassic-mammaliamorph-with-three-dozen-perinatal-young
https://www.nature.com/articles/s41586-018-0441-3?utm_medium=affiliate&utm_source=commission_junction&utm_campaign=CONR_PF018_ECOM_GL_PHSS_ALWYS_DEEPLINK&utm_content=textlink&utm_term=PID100062364&CJEVENT=2b050606934b11ed8085004d0a18b8fb
https://www.sciencedirect.com/science/article/abs/pii/S0143400409004135
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4284630/

Видео Evolution of Live birth (Why do mammal not lay eggs?) канала Moth Light Media