What makes you think nobody "stepped up" on Android? There are a number of Instapaper clients for sale in the Android Market. I've only tried one (which sounded like the best from reviews I read), InstaFetch, and it's quite good. It has a couple of extra features beyond Marco's client: it can read articles to you (Android comes with a TTS library, I don't know if recent iOS versions let developers use Siri's TTS function from their code --of course, if Marco wanted to add TTS he could license a library or something like that, but it's definitely not as convenient for him as if he could just use a system API), and it updates your articles without you having to run the app (which I understand was not possible in iOS [1], but again, I wouldn't know if a recent version changed that). Plus, with Androids intent system, you don't need to install the send-to-instapaper bookmarklet in the browser: InstaFetch adds an add-to-instafetch intent that you appears in the browser's menu of ways to "share" the page .
The one thing Instapaper 3.0 did really better than InstaFetch is that it's option for changing from the light to dark theme is accessible right from the article view, while in InstaFetch you have to go to settings.
[1]: http://www.marco.org/2010/06/10/iphone-multitasking-and-back...