Ask HN: Why is crawling/indexing SPAs more difficult than server rendered sites

This is a slightly less touchy / controversial question than my last one. Also I am a mobile developer but sometimes I like to explore the web development scene so this might seem like a silly noob question.

Why can't web crawlers index websites as efficiently as a website that is the traditional server side rendered website?

Is it just a matter of inefficiency for the crawler to wait for JS to load the UI? Aside from doing some kind of hybrid approach of client-server rendering, what can web apps that are SPA do to increase visibility?