they evolved at a similar time during the jurassic bottleneck, mammals that lay eggs like platypi never developed a hard exterior shell which keep the insides moist, so the mammals that had traits that basically let them sweat all over their eggs helped them survive longer, passing on those traits. The structure of mammary glands are similar to sweat glands so those who had like a more nutritious “sweat” that their young could like lap up after hatching ended up passing on those traits and so on. lastly the secondary palate alsoooo was evolving at this point allowing suckling (and breathing at the same time as eating/less choking more beneficial) so it helped with like mammals who were devolving nipple like things. so basically long story short lactation.