It's especially important to understand that in the early days of computing you only had a single system font or a tiny font pack of less than 10. Microsoft used a tiny pack that had Arial as their sans-serif, Times New Roman as their serif, Courier New as their monospace, and two symbol fonts. Of the three real fonts, two were incredibly computery, leaving Times New Roman as the friendliest font available.

Microsoft then set out to make a family-friendly cartoon-themed OS and set the entire OS in TNR. One of their designers took issue with it and designed Comic Sans to be a viable OS font that felt friendly and suited comic art. He was a bit late in starting, so it shipped with TNR and immediately failed, but Comic Sans ended up shipping with some versions of 95, then became a default font in Internet Explorer and Win98. As a result, it was the only friendly font on the average computer for ages.