Bing is hardly a failure, nearly every search engine that isn't Google has switched to it as a data source, and it's market share is growing. Bing's at a third of US search (according to Microsoft, at least): https://arstechnica.com/gadgets/2017/08/bing-is-bigger-than-...
A lot of companies now compete with Google in one or several markets, and being forced to include Google Search would be fairly painful for a lot of them, so Bing tends to be an immediate contender for anyone who isn't Google trying to integrate search capabilities. Obviously, a fair segment of their market share is just "Bing is default on Edge/IE, and Edge/IE is default on Windows" as well.
Baidu is pretty much limited to a single country, mind you. And Yahoo uses Bing data, and almost certainly pays Microsoft for it.
Microsoft almost certainly spends a lot less money on Bing than Google spends on Google Search, there's a good chance that Microsoft's spending per search query is lower than Google's. Bear in mind, being "good enough" costs significantly less than being "the best".
I hate being depended on Google and I tried hard to break its chains. I can't. Neither ddg, nor Bing, nor Yandex are anywhere close to Google search quality. I'm periodically trying them, but it seems like Google is light years ahead. And using Google Search is as integral to my work as using IDE, so switching to something like Bing is the same like switching to Notepad.
I haven't used Google in years. Haven't had an issue that Bing couldn't find that Google could. FWIW, you can use Startpage to get Google results without feeding the beast, or use the !g command when searching with DDG to get Google results.
A lot of companies now compete with Google in one or several markets, and being forced to include Google Search would be fairly painful for a lot of them, so Bing tends to be an immediate contender for anyone who isn't Google trying to integrate search capabilities. Obviously, a fair segment of their market share is just "Bing is default on Edge/IE, and Edge/IE is default on Windows" as well.