Because you can just resize the IR window in Max, making it as small as you like using the usual windows resizing:) This is not possible in C4D, the window will by default be the same size as your final render and can't be made smaller, so the addition of this max resolution was introduced to circumvent that.
(EDIT - Also, I don't think the Max IR window ever gets larger than one monitor, unless you scale it up yourself, while in C4D without this parameter, the IR will open at 8K if you are doing an 8K render, all very awkward without this extra parameter.)