Garbage Collection. Not that long ago Apple came out with full GC in Objective-C to a lot of fanfare (no more retain releases yay). Not long after that they said to just use ARC (which is compiler retain/releases) because the GC was not working well. Today, it's hard to find any mention of the GC.

The GC had at least one fundamental flaw:

If your app used GC then everything your app links against or loads at runtime has to be GC-compatible. So Apple had to make all of it's frameworks GC-compatible. But many 3rd-party frameworks didn't.

Also: If your app used WebView to load a web site which contained something with flash (YouTube, ...) then your GC app would crash immediately because WebView loaded the Flash plugin which was not GC-compatible.

Thus using a WebView to load arbitrary sites was incompatible with GC.