There's a big difference between Macs and PCs you should take into consideration.
Most of PCs are sold for businesses, and the main user has few privileges on what to install and update. Tech services handle the big updates.
In business, the rule is "if it ain't broke, don't fix it" and the same rule applied by contamination to many other PC users. When a PC is sold with XP, keep on using XP until the PC is broken, or when XP gets old, wait until Microsoft releases a new Windows version and we'll get new PCs with the new release.
That's why XP survived that long and PC sales were in trouble during the Vista era. Many companies didn't want to get new PCs running Vista. They stuck to XP and harrassed Microsoft to keep on selling volume licenses of XP. When W7 was released, business eventually realized it was a significant improvement and renewed their computer stock with new models running Seven. But few personal users felt the need to buy the upgrade disk for their shop. It's not much part of the Windows culture.
The situation is quite different on the Mac. Most of Macs are home computers that are administered by their main user. From Mac OS X 10.0 to 10.6, there were significant progresses for features and performances (10.0 to 10.2 were slow and not very comfortable), and zero piracy protection, so most of the people decided to upgrade, legally or not, as we got a new and definitely release every year or every two years. Mac OS X 10.7 was some kind of a departure, and it's much slower on some models, but Apple has resumed their policy of refining and enhancing up the OS major releases, this time with a new release every year sold for a pittance (less than $20) on the Mac App Store.
And that's why almost every Mac in use today runs on 10.6 (last satisfying release for early Intel Macs), 10.7 (for models that aren't fully 64 bit compliant) or, in majority, 10.8, while in the Windows world it took years for Seven to rise above XP. Mac users tend to take a chance on change and progress much more than their Windows counterpart, mostly as Windows is that much tied to businesses and businesses are usually on the safe side.
So, in the OS X developing world, nobody would care now for 10.5 Leopard, even less 10.4 Tiger. The oldest version commonly in use is 10.6 Snow Leopard, and it was released in August 2009 just a few weeks before Windows Seven. That's as if Microsoft could afford to drop all support on XP and Vista and introduce bigger changes in the architecture with a major release, knowing that developers and users would be quick to follow them. They actually attempted this with Windows 8 (and Windows RT) and it flopped badly.
On iOS, it's even simpler. 93 percent of the devices that go on the App Store run iOS 6. 6 percent are on iOS 5. If you want to develop on iOS, focus on the latest version, if you're on the cautious side, support the version that was released the year before, and you cover 99 percent of your users. No need for compatibility with older APIs. When new features and APIs get introduced with major releases, you won't lose anything for taking advantage of them. It will actually put you above your competitors.
I don't write this to say that Mac or iOS is better, especially for developers. It's just that the culture of upgrade on Mac and PC is significantly different due to their respective markets and traditions. If you want to develop (for OS X or iOS) on a Mac, you have to know how Mac users (and common developers) behave compared to their PC counterparts, and get an OS upgrade when the early bugs for a major release get eventually addressed.