When all this stuff was originally decided, clearly the British Commonwealth currencies were ranked as most important, which is why GBP and AUD are generally the base currency. USD slotted in to the second position. At that time, of course, there was no EUR. The European continental currencies were always listed as the non-base one against GBP, AUD, and USD, but were the base against the JPY. I can't say if that was a conscious decision made somewhere back in time, or if those conventions just sort of developed over time.
I wasn't around for the earlier decision, but the one to make EUR the base currency in all pair quote was a conscious one by market participants. Some folks disagreed at the time, but it was (I think) based on the size of the market for Euros as compared to other currencies. Since there are now more Euros in circulation than any other currency, seems like they made a decent decision.
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