I don't know the answer to your question, I can offer an educated guess.
I suspect that when those words were drafted - probably in the 1940s or 1950s, the difference between CAS and EAS was trivial and not considered as important by anybody.
Over the 1950s and 1960s aeroplane performance became greater, and engineers - and eventually pilots - were required to start including compressibility corrections and allowing for the difference between the two. However, I don't think that the regulations ever caught up.
But, realistically, does it actually matter? We will always need a quantity called IAS: that which is indicated in the cockpit, and that's what aeroplanes will be flown to. TAS will always be important for navigation calculations, and some aerodynamics. EAS will always matter to engineers, but never really matter to pilots other than as an intermediate quantity.
So, is there really an issue in having CAS - which just ensures that at each step from IAS to G/S there's a single set of corrections, rather than two corrections.
G