There was animosity between Steve Jobs and the Lisa team (who perhaps not coincidentally chose to name the system after his daughter). Once he decided that the Mac would compete against the Lisa, the Lisa platform was doomed. Jobs basically told customers, software developers, and the press that the Lisa was obsolete because the Mac was coming out soon and would be cheaper and better. He was correct about the cheaper part.

Unfortunately the Mac cut a lot of corners for affordability. The original Mac had only 128K of RAM, and Jobs didn't want to offer memory upgrades (he thought you should just buy a new computer - sound familiar?) It took Mac OS 16 years to get memory protection, which LisaOS had in 1983. Lisa didn't need to die - it could have merged with the Mac and made the latter a better and more reliable platform, years before Mac OS X.