Because it's not true that Apple doesn't emphasize backwards compatibility. I also pointed out that once those technologies outlived their usefulness, they were no longer supported, and that there is indeed less backwards compatibility than Windows.
By the way, Office 2008 ran on Intel Macs. You're thinking of Office 2004, which was PowerPC only and ran for 7 years before OS X Lion dropped Rosetta.
Macs transitioned to x86 in 2005, with Apple repeatedly warning developers to GTFO of any PPC code they have ASAP. MS Office 2008 released years later still containing PPC binaries, and support from the OS was finally yanked in 2011, six years after the switchover.
This seems pretty reasonable. What is unreasonable is that MSFT continued shipping deprecated code as the newest and greatest, long after any reasonable update cycle.
To be fair to Microsoft, they weren't the only ones. App developers were warned for years since OSX's first release that Carbon was a transitional API that would be deprecated in favor of Cocoa. This was in 2001. Adobe steadfastly refused to update, and in 2007 Apple stripped 64-bit support from Carbon-based apps as a beating stick to get major developers like Adobe moving. It worked.
From everything I've seen developing both OSX and iOS apps, Apple gives plenty of notice between deprecating functionality and actual removal of binary support (from the above examples, six years).
Compare with my days developing Windows software, where developers knew well that Microsoft had zero teeth behind API deprecations, and will shamelessly keep using it for eternity, making future backwards compatibility efforts ever more painful - you will find code written in 2008 that uses functionality deprecated in the late 90s!
Then, IBM bought Transitive and discontinued the PPC->Intel translation engine.
Apple did not have the option to continue shipping Rosetta, because IBM axed the team that would have had to update it for Lion.
To support only applications that existed prior to the Intel switch, they wouldn't need to back-port new APIs.