It's like this:
1) Armalite designs the AR-10 around the 7.62 NATO cartridge using the latest in aerospace tech. They also work on a design that is better able to manage recoil despite the lighter weight. The AR-10 evolved and parts where redesigned for better usability and beefed up where needed; a few foreign countries adopted the updated versions.
2) The medium cartridge idea was adopted and the AR-10 was down sized into the AR-15. Of course, weight was further reduced with the smaller .223 round and the AR-15 evolved just like before. Parts where beefed up where needed, the layout evolved into today's format, parts where simplified for mass production, etc. The final AR-15/M16 was standardized and went into service and continued it's evolution from E1, A1, A2, A4, M4 and so on.
3) A couple of companies in the 90s decided to reintroduce the .308 AR. Eagle Arms bought the Armalite name and remarketed the "AR-10". This AR-10 was a scaled up AR-15 and attempts to use as many AR-15 parts as possible to reduce cost. It, the "B" model, shared only the .308 chambering of the original AR-10 until the "A" model came out using the original pattern mags. M14 mags were chosen as the first mag pattern because real AR-10 mags were ultra rare, ultra expensive and due to the `94 Klinton AWB, illegal to make in standard capacity for civilians. With it's expiration, new standard capacity mags could be made and the Gen II AR-10 mags were released followed by a original AR-10 pattern mag option.
Knight's Armament didn't care about such issues as their efforts were LE and .mil related, and with Stoner designed the SR-25. Stoner used his original mags because, surprise, they worked best in the platform. There are few parts that carried over from the AR-15 to the SR-25. With the expiration of the `94 AWB new standard capacity SR-25 mags could be bought by us civilians. Other companies designed rifles around the SR-25 mag as again they worked best, the SR-25 was adopted by the military, and they weren't patented by Armalite. Of course the price of KAC mags turned people away and DPMS couldn't make a 20 round mag that actually holds 20 rounds, so Magpul was able to come in and offer their P-mag in SR-25 pattern. Now you have a proven, reliable, inexpensive mag; plus the SR-25 pattern mag is the adopted pattern by the .mil and everyone is going that way.
Unfortunately the .308 pattern AR is not standardized like the AR-15 so despite using the same mag, very few other parts interchange; and NONE of the parts interchange with the original AR-10.
