because it is Lisa Simpsons name.

Lisa started out as a European diminutive of Elisabeth/Elisabetta etc. It may owe some of its popularity to the "Mona Lisa" painted by Leonardo DaVinci, the name of arguably the most famous painting in the world. It was hugely popular in the USA in the 1960s and 1970s, and has since tapered off drastically in popularity.
See Also: Elisabeth

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