We were told in several occasions (speech, CO visit, assemblies) that as witnesses were (back then) over 6 millions the attendant count in memorial showed the true immediate potential. It would have been about the double amount witnesses if all those in memorial audience could be converted (sorry that word was forbidden). If they cared so much that they attended memorial it shouldn't be that difficult to encourage them to other meetings too.
I think that was very easy way to tell the people that we are going to right direction, look what we could achieve if we just would do even more and better. If memorial attendance would have been the same as in regular meeting it could have led people thinking what is the point.
The eager interested ones you could attract to regular meetings, but only other occasions where you could show your interested studies or magazine routes physically were actually memorial and assemblies in some extent.
CP