Well, microsoft isn't going to develop for previous gen iOS, why would they? iOS 7 is much different than iOS 6 and the majority of ios users are on iOS 7 now.
Let me chime in as a developer with several apps in the AppStore, all supporting all iOS versions starting with 4.0 (with armv6 support, meaning I do support the iPhone 3G and the iPod touch 2G in even my latest apps I've submitted 1-2 weeks ago).
It's pretty easy to make an app support even old iOS versions. If MS uses autolayout (I don't know) or, NSMutableAttributedString's addAttribute:value:range (something quite useful with rendering showing formatted HTML in a lightweight component - that is, not in a UIWebView - and consequenltly probably used by their suite) then, the app could quite easily be compatible with iOS6 too, covering a lot more devices.
Without those features, even iOS4/5 support could be possible, depending on other features.
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As far as Apps needing iOS7. I feel like i have seen quite a few. That is the way developers will be doing things...releasing for the current OS and not worrying about the past. this means they can use the latest SDK and latest stuff Apple has added. There won't be a need to make sure you have 2 things going at once.
That's definitely lazyness on the devs' part, unless they really use some non-existing stuff. I generally only use iOS7-only stuff in conditional blocks and try to come up with something similar / workaround in the code targeted at earlier OS'es.
For example, I use the above-mentioned
[NSMutableAttributedString addAttribute:value:range] in one of my apps to display simple, random HTML formatted text in a table cell. As formatting isn't THAT important, I simply display the text as is (w/o bold / italic / etc. formatting) under pre-iOS6 OS versions. The users don't lose THAT much - certainly this is much more a user-friendly solution than not making the app compatible with iOS versions prior to iOS6 at all.