Because it is the truth.
I think you don't understand what I wanted to tell you. Of course he became stronger than in Crisis Core. That's fact and everyone knows that. But even though there is a Fan Club question about him he cannot use that move during that time for real - otherwise you would say Super Nova would look different in Crisis Core compared to Super Nova in Final Fantasy VII but this argument would make no sense because he doesn't use Super Nova during that time and so it is pretty logical he simply cannot use this move because he is not strong enough during that time.
Yes, they got stronger but even directly before the Final Battle they have said they are weaker. And it was shown by the same way Square has shown it with Kefka. Kefka has pushed the characters around with his power and Sephiroth has forced them back with his power. Of course it was teamwork, they were the strongest heroes, et cetera but the characters were still weaker. You do not have to be stronger than someone to beat him. That's the same in every other topic. Just because I beat...a World Champion in a trading card game it doesn't mean I am better than him in this game. I could have been luck. Or bad luck for him. There are so many factors. And as Square says Sephiroth is the strongest and he wasn't even at his limits in Advent Children (I know we are talking about Final Fantasy VII-1) while Cloud was (and we can agree Cloud - and Vincent - are the strongest characters of the hero team, compared to them the other's power hardly counts). So that shows no matter how strong the characters are they are not stronger than him. Because it is not necessary. Sephiroth has his arrogance weakness.
This is just your opinion and can't be really proven. Like my explanation about why Super Nova is a power demonstration cannot be 100 % proven without Square as well.
I have to disappoint you but he didn't.
Cloud himself mentions Sephiroth could sacrifice one of his other clones to get the Black Materia. He has said in in the Temple of the Cetra. So he could sacrifice another clone as well who would bring him the Black Materia - because of Final Fantasy VII's definition of "clone" Cloud is a clone of course because he got J-S-cells injected but I think you've understood what I have meant. Cloud was not necessary. There were enough clones. And they have followed the desire to reunite (activated by Sephiroth with JENOVA's head) as well so they would have wandered to the Northern Crater as well (and they did like we all now). Sephiroth could have one clone to activate the trap inside the temple and another one could have brought him the Black Materia. Cloud was not necessary and so it is a filler.
It doesn't explain why Super Nova should be the only move in Final Fantasy who doesn't show what a character can do. Let us think for just 1 minute you are right (and we will never know), it is fact Sephiroth can use a move called "Super Nova" - why should it look like a Super Nova if he cannot unleash a real Super Nova? This makes no sense. He doesn't have to impress other people. People already fear his power.
Well - now really that's it. This is just a "he can" and "he can't". We don't get far so let's spend our time useful.
And Khalin just because you don't like Dragon Ball the examples are not invalid. They have the power and that's it. I know another fantasy- and anime-like character who is imcomparably strong and is probably the best example. He is not of an anime itself. But he is anime-like as well. One of the well-known forum members here is called like him.