Only 5 slots in Heroes 1 and 2 - Why?

I just played Heroes 2 again for nostalgias sake and then I remembered something I've always been wondering about. Why on earth did the game developers only make 5 slots for creatures in your Heroes army when you can build up to a total of 6 creatures?

It never made sense to me that I had to remove a creature from the army, preferably the lowest tier creature after obtaining all of the creatures. In Heroes 3 and the rest of the series, you could include all creatures. And so, my theory is that this was a predesign planning thing in which they later figured out that 6 creatures would take up too much space on the battlefield. And so they had to narrow it down to only 5 creatures. I can't really see any other reason because it's quite stupid not being able to recruit your whole army and go to war with it. I especially hated this in Heroes 2 and never understood the reason why.

Thoughts?