why the "replace" is not long sufficient?

in the vbseo previous versions, i was need to only check the box "overwrite the existing setting in add/import product,
why in this newest versions i need to uninstall vbseo before the upgrade?

Thanks in advance, it is not a problem for me, it is only a my curiosity.

It's more related to vb than to vbseo. vB has re-done the entire product system making all products not comaptible between the two versions, hence you must uninstall (or at least disable) before upgrading.

vBSEO doesn't need to be uninstalled. You only have to deactivate vBSEO prior upgrading vBulletin.