Limited Hero Spirit Spots

Why is there a limit on Hero Spirit spots for upgrading your current Hero Spirits? It also seems to have a lessening effect on the number of spots available, as the more Hero Spirits that you have. At current, I have 31 Heroes with each having a Hero Spirit. When I select to have more for upgrading purposes, I can clear the history twice and get almost halfway on the third, showing about 2 & 1/3 pages on the enhance page or 3 pages & 6 slots on the Hero Spirits Backpack. Was there a limited cache built in, which gets smaller and smaller with the more golds and purples you have? Because it is getting annoying already with no visual representation as to why it is this way.