Law [of] merchant

When I first met the term "law merchant" in a book I was using for our negotiable instruments law class I immediately listed it as a typo at the back of the book. I thought the proper term would be "law of merchant". Later I learned that indeed it is "law merchant", not "law of merchant."

Then I reached page 223 of The Essentials of Transportation and Public Utilities, 2011 Edition by Aquino and Hernando and read this:

So I thought that perhaps it should be law of merchant if Lord Justice Bowen indeed stated it that way. But googling the source (see here) revealed:

Thus "law merchant" it still should be. Even Aquino and Hernando got it wrong.

But is there any reason for the use of the term when "law of merchant" will arrive at the same meaning and is less stilted? I think the reason is that "law merchant" is a word for word translation of "lex mercatoria" which means "merchant law".

This reminds me of an earlier post (see here) where I explained why the Civil Code uses price certain instead of the straightforward certain price.