Probably due to the fact that there are not only multiple wireless charging standards, but each phone vendor coughSamsungcough wants to make their own pointless proprietary variation on it. As usual, once Apple does it all the other vendors will follow suit and it will become the standard.
For the first bolded part, realistically, there are only 2 main standards. There are other technologies but there is no widespread adoption of them.
For the second bolded part, devices like Samsung's support both. Your claim about a proprietary setup is incorrect. They implemented fast wireless charging (still using Qi). The industry in general had not adopted fast wireless charging till Samsung pushed it and now there are 10's of million of fast charging Qi/PMA capable devices everywhere that still work perfectly fine with regular wireless charges. The technology was not proprietary.
For the third bolded part, this is possible, but ideally whatever Apple implements supports the current established standards as well as whatever they decide to push. The rumors are getting stronger that the new iPhone will support wireless charging. It remains to be seen what it is.
As we have seen with USB C and Lightning, while Lightning is widespread, even Apple is moving to USB C to consolidate components.