In a recent blog post, Microsoft announced a new version of Outlook (the email client for Mac), but at the same time pushes back the launch of a new Mac version of its Office suite (Excel, Word, and of course PowerPoint) by a year to the second half of 2015:
Historically we have released a new version of Office for Mac approximately six to eight months after Office for Windows. However, following the release of Office 365 we made the conscious decision to prioritize mobile first and cloud first scenarios for an increasing number of people who are getting things done on-the-go more frequently. This meant delivering and continuing to improve Office on a variety phones (iPhone, Windows Phone, and Android) and tablets (iPad and Windows)—brought together by the cloud (OneDrive) to help people stay better organized and get things done with greater efficiency at work, school, home and everywhere between.
Continuing our commitment to our valued Mac customers, we are pleased to disclose the roadmap for the next version of Office for Mac—including Word for Mac, Excel for Mac, PowerPoint for Mac and OneNote for Mac.
In the first half of 2015 we will release a public beta for the next version of Office for Mac, and in the second half of 2015 we will make the final release available. Office 365 commercial and consumer subscribers will get the next version at no additional cost, and we will release a perpetual license of Office for Mac in the same timeframe. 
Microsoft is prioritising mobile over its desktop software. It is true that mobile is the hot area right now. But - call me conservative - I still think that initial content creation and design still will require a desktop machine with a big screen. A quick reply to an email, a Tweet, a quick presentation edit, can all be done on a mobile device. Great design, great stories originate from someone focussing, rather than boarding a train.

The other reason behind the delay might be that Microsoft is simply running out of ideas what to add to PowerPoint. It is probably true, that the software has been pushed to its limits, and that the true innovation lies in what to take out. I am working on it.