Conventional oil is still around because it's cheap, and because it's good enough to cover the needs of a lot of engines. Don't forget, most cars on the road are economy cars or fleet vehicles. Those applications have strong demand for cheap and forgiving maintenance regimes. Automakers know pretty well how to make that work, so there are a lot of engines for those applications that can run all day on a conventional oil.
Oils have gotten better, too. A conventional oil that meets modern specs is pretty robust.
There are also a lot of older vehicles on the road that are operated by people who can't or won't spend much on maintenance. Those folks create a huge market for oil that doesn't cost the better part of $10/qt.
Lastly, there are some weird applications, like Mazda rotary engines, which Mazda recommends NOT to use synthetics or blends in.
As thinner oils (xw-20 and below) become more prevalent, we'll be seeing fewer straight-up conventionals, and more synthetics and blends. It's hard to make a conventional oil that thin that won't cook off too easily. But conventional oils will be around for a while yet.