I'm curious why you feel syncing third-party devices should be part of the operating system?
It's not feasible, or technically possible for Apple to obtain every third-party mobile phone, and write software that supports the thousands of variations in mobile OSs and sync methods.
Like printers, scanners, digital cameras and other electronic accessories, the manufacturers of those devices are the best placed to provide the best support for those devices, especially when released. It would take Apple much longer to 'reverse-engineer' or otherwise obtain the required technical details to provide 100% support for every device specific feature.
Also, iSync only ever supported 'feature phones' - not smart phones. All the world's major phone manufacturers are quickly moving away from making basic phones, so as time goes on, iSync would be less and less useful.
Finally, from a purely business perspective, why would Apple spend valuable time and resources supporting other manufacturers phones for free, when they would prefer you bought an Apple device? Microsoft don't have OS level support built-in for syncing non-Microsoft OS phones either.