The answer to this question is pretty simple. The main reason HO Scale wasn't adopted as the universal scale for all model railroads was that it became too expensive to produce models in HO Scale due to the need for smaller equipment and more precise measurements. For example, a locomotive that was three feet long would be about one inch long on O Scale because of how many times larger the O Scale is than HO Scale. This means that even if you had an entire train set up on your table-top, it would still take up less space than a book sitting next to it!