One of the main excuses given for not enabling HTTP 1.1 pipelining was a small amount of software that didn't handle it correctly. According to Google, we needed a new protocol so we could start out fresh where there wouldn't be software not implementing the protocol correctly.
But according to this report, the browser with the largest market share and by the creators of HTTP/2 had a bad implementation of it for which a workaround had to be found for.
For history's sake we should recognize now that this buggy software argument for not enabling pipelining was never valid. Not enabling pipelining was used as a political tool to prevent HTTP 1.1 from being on par performance-wise with the new protocol people wanted for other reasons, and that's also why Google never even once tested SPDY against pipelining.