Because Microsoft is coming for them, targeting one of Adobe's flagship products.
Later in the day, Microsoft also unveiled "Metro," which can be best described as an alternative to or replacement for Adobe's PDF. Metro is an XML-based document specification that covers creation, viewing, and printing. Windows applications will be able to create Metro documents and Microsoft hopes to have printer manufacturers include support for it in much the same way as Adobe's PostScript. Microsoft will offer developers APIs so they can incorporate its features into their applications, as well as royalty-free licenses in order to encourage its adoption.
Adobe broadened their portfolio, so to speak, just in case. I wonder what they know about that hasn't been announced by Microsoft yet.
[Via Ars Technica]