Only a speculation, but I am afraid it will be hard to be more certain without Bjarne Stroustrup confirming how it really was:
It was designed this way because it was assumed such behaviour will be expected by the programmer, that he will expect the copy to be done when = sign is used, and not done with the direct initializer syntax.
I think the possible copy elision was only added in later versions of the standard, but I am not sure – this is something somebody may be able to tell certainly by checking the standard history.