To be fair, if you compare IE11's with other browsers at the time of its release, they have stellar support. There is still no other stable browser that supports let, and the only other browser with support for const is Firefox 36 (released yesterday).
For better or worse, Internet Explorer has a far longer release cycle. But right after release IE is often ahead of the other browsers in some regard (CSS calc and the ECMA Internationalization API come to mind).
The kindest thing you can say about IE is that they seem to be well aware of the problems with their release cycle, and are taking steps to remediate this -- they refactored out large amounts of compatibility code from Trident to make EdgeHTML, and I think they plan to up the release cadence of updates to EdgeHTML significantly once they release Spartan.
Apple doesn't seem to be doing much of anything to keep up with Firefox and Chrome on those charts. Which isn't even the problem! Apple will not allow Firefox or Chrome's rendering engines onto iOS. Even in the darkest days of the IE/Netscape war, Microsoft never even contemplated not letting users install Netscape.
It's the combination of "lagging behind standards" and "wholly locked-down platform" that threatens to stagnate the web, and it's only Apple that's in that position.
The actual work is going on here, but it sounds like it's only at the experimental stage: https://github.com/mozilla/firefox-ios/ I'm kindof curious to build it and see if it works.
I read that the only issue with getting competing browsers onto iOS is the developer agreement says you can't provide a program with a proprietary javascript engine in it. Frankly, iOS is so big now, I would argue that that restriction strays into competitive advantage territory, and the OP is proof of that. If I were a lawyer at Apple, I would suddenly become magnanimous and get the developer agreement changed such that competing web browser programs _only_ may host their own JavaScript engines.
It is "Firefox", but it's still using the Safari rendering and JavaScript engine under the hood (like Chrome for iOS does), because there isn't an option to do otherwise.
The first problem is political - firstly Apple make it so that you have to use the Webkit rendering engine for any third party browser.
The second problem is more technical - iOS has memory protection that means you can't take a writeable page and make it executable. This means that you can't (for example) JIT compile Javascript (or any other language). Their built in exception to this is Safari. This is why Safari is faster than webviews.
For better or worse, Internet Explorer has a far longer release cycle. But right after release IE is often ahead of the other browsers in some regard (CSS calc and the ECMA Internationalization API come to mind).
Safari hasn't done that in a while.