I think I can answer 1/2 of your question. The 270, 7mm and 300 WSM were supposed to be introduced at the same time. Winchester discovered some problems with the 7mm version and at the last minute decided to tweak the design some before introducing it about 1 year later. By then the 270 and 300 were already established and the 7mm never caught on. Which is really a shame as it might be the best of the 3.
As to the 325 WSM. That is a huge mystery to me. It still isn't very popular and I expect it to eventually die too, but for now it is hanging on. It shoots the same bullet weights to virtually the same speed as the 300 WSM. At best it might better 300 WSM's by 25-50 fps, but the much better SD and BC's of the 308 bullets easily match or outperform the 325's at any range. You can't convince me that .015" greater bullet diameter is of any significance. After bullets hit and expand no one could tell the difference in expanded bullet diameter between .308 and .323 bullets.
