I wonder why they didn't design it as a separate program to play this type of file?

Obviously, the author would be the only one who could answer your question, but I have an assumption that may have some truth to it. The demoscene is not only a community of friendship, but a community of competition, too. And when you compete, you may very well try to keep your advantages to yourself or, at the least, try not to draw the attention of your competitors to it. Failure to do so resulted in e.g. the Commodore 16/+4 scene in certain cases in animosity between the sceners and some exaggerated measures.