Total guess: maybe there's a greater chance of exit for very early stage (easy acquisition target) or very late stage (IPO). A lot of the failures that make the news are companies that hit the Series B or C phase and realized they had no path to IPO or acquisition.

It could be that post-series-B companies are less attractive acquisition targets. I don't have any data on this.

Quite possible that the really good companies don't need to raise after Series A