Because these two companies make their money off downloads. everything that xbox offers costs money in some way, i cannot even join a chat party without xbox old. I think they simple looked at it as "if we let people create content we could be missing out on DLC money".
While Microsoft most assuredly makes money off DLC, and even FORCES costs onto packages beyond a certain filesize threshold, Sony has yet to show anything that would give that statement credence. There are far, FAR, more free download packs on PSN than there are on XBL.
LittleBigPlanet, which is a 1st party Sony game, allows for user made creations free of charge. Halo 3, which is a Microsoft owned title, allows for custom maps and gamemodes... of sorts. These are also free of charge. And before you go off on a tangent about how the creation tools for these games are different, NO. NO THEY ARE NOT. You can do pretty much the same things you're allowed to do in those games as you can with the ES Construction Kit. Even on the PC, you can't create an entire texture or model pack using just the CK.
So I hardly think them denying the CK (which AFAIK, never happened anyway) to be put on their systems purely for money is a bogus claim.