As a developer adding and testing a new building block version is cumbersome as an existing building block can not be "upgraded" into the draft version. Currently, especially if its a "apply once" building block you need to create a new tenant/project to apply it or remove the old one and re-apply the new building block as draft. This might not always be easily possible.
It also only allows testing the "upgrade" procedure once the building block is released and an admin actually performs the upgrade into the new version. If you could already "upgrade" into a draft version this transition could be tested beforehand.