Whereas most websites and online web services must continually update and improve user design, Craigslist, a classified advertisements website, has remained largely the same since 1996. What phenomenon might explain this?

A network effect that makes switching to competitor site difficult.

Explanation:

Whereas most websites and online web services must continually update and improve user design, Craigslist, a classified advertisements website, has remained largely the same since 1996. the phenomenon might explain this is a network effect that makes switching to competitor site difficult.

network effect is a phenomenon whereby increased numbers of people or participants improve the value of a good or service

Craigslist since it inception has a massive international user base, gained popularity as a text based, it had more sellers and buyers than other sites, making it the most attractive site to use regardless of whether it had been updated and improved, as it was well organized and easy to navigate and functional. To put it simply, it works and accomplishes the goals of its users without ads popping up. Therefore, it literally doesn't need to updateand improve it user design.