I think we can safely say that "web page don't go down anymore" is not the real reason. Two possibilities I see for the real reason:

• They don't feel it's necessary anymore to cache the actual pages. It's not productive to have them to track and analyze SEO abuse, or even just to tune their algorithm by rescanning things if someone notices there's a problem, even given that it costs them a fraction of a fraction of a fraction of the income they make from search, and it makes life easier for anyone who's working on it.
• They used to make it available just because it was a neat thing to be able to offer and there was no reason not to, since they were caching the pages anyway. But, the current Google management has wandered so far afield of the original mindset that made them successful in the first place that that means nothing to them, more like a bizarre confusion of concepts than any kind of statement that makes coherent sense. So fuck the users. Yes we're still caching them, because the search engine needs them. No you can't have them for free, you fucking hippie. Now get out.

I know which explanation I favor.