It's actually stated in the LoS enemy profiles (in the menu) that while on the mortal plane he is weaker. That, plus the Uber Holy satellite death-ray from above (or whatever it was) that he gets blasted with while being constrained by Gabriel, is partially what knocks him down for the count for almost 1000 years. Another reason to wait so long to enact revenge, he wants to make sure he's powerful enough to defeat Gabriel this time around. Considering how powerful Gabula is now (or was), after having absorbed all of TFO's power, Lucifer is probably not taking any chances.
Well, that's how I read it at least.