It's a good question, unfortunately I don't think there has ever been an official statement as to why. Perhaps it's just the fact (speculation) that by the time the game was available for steam distribution, the editor no longer existed in it's original form (being redesigned for future titles) with Bethesda.
Bethesda has always been a little weird when it comes to supporting and maintaining peripheral assets (dlc, their editors, etc.). This is the same company that can release a major title through steam, setup a modding "workshop" with the same, yet still not have Oblivion's dlc available on their own web store. Strange, yes?
If Bethesda has no interest in supporting past titles, then I say hand over the game editors to the community. I afraid that we could lose the next generation of modders, and all the goodies that they can create.
You could try and PM Gstaff or customer support with Bethesda, but I doubt it will get very far.