The title says it all. Because old versions should also be involved.

Take this run, for example:

It is ran in an old version of the game. (Version 0.7.4). But we can't just say it is invalid and throw it away because it is a very good run!

Having separate, unfinished versions of the game on leaderboards would get way too cluttered very fast. Also, these versions of the game are no longer available to the public. only people that downloaded the game from itch.io prior to updates could run them. Considering that the current version is overall a stronger speedrun than past versions, there isn't enough incentive to make a leaderboard for them, even if they are interesting historically.

Disclaimer: I did runs of early access versions long ago myself. Here's a run on 0.7.3