Microsoft got flack for dropping 16bit app support in 64bit OSes (since 64 bit windows doesn't have NTVDM). They'd be flayed alive if they dropped 32bit support.
Crucially, however, 32-bit Mac OS X 10.5 could run 64-bit apps just fine. That, combined with OS X's universal binary support, made it easy to start deploying 64-bit application code far earlier than most Windows apps began transitioning.
Most of Apple's computer models were only available with 32-bit x86 for less than a year, so the installed base that needed their applications to still be 32-bit was minuscule compared to the installed base of 32-bit Windows editions that couldn't run 64-bit apps even if the CPU was 64-bit capable. Most Windows application developers faced bigger challenges in deploying 64-bit code than Mac app developers, and the Windows devs' work would benefit a much smaller fraction of their userbase. Windows app developers don't switch to 64-bit unless they absolutely have to. (e.g. this year's 64-bit re-release of Skyrim, to accommodate the address space requirements of heavy modding. Skyrim was originally released in late 2011 as a 32-bit only game despite listing CPU requirements that couldn't be satisfied by any 32-bit CPU.)