Because, really, what would Grandma gain from switching?
Why should Grandma use Linux?
Grandma would gain nothing. But I certainly would, because then I wouldn't have to reformat/reinstall her OS every few months, and wouldn't have to bother about malware infestations.
I can't hope to educate grandma about best security practices, so I'll do the next best thing.
I'll install a solid distro with all the bells and whistles she needs (it may be ubuntu or it may be slackware, won't matter, she'll only be using some programs), customise the GUI to be as simple as possible to meet her simple needs, disable all unnecessary ports and services, configure ssh and port 22 (just in case she asks me to do something later), turn on monthly updates, and be done with it for some years.