The reason 5nl is harder for you because the players don't
bluff as much. 25nl players bluff more. 25nl is actually a harder game to win at, but at 5nl it can feel more like a grind. People at 5nl and 2nl wait for the nuts and seem to only bet with the goods. You are probably calling people down too much if I was to take a guess.
You still need to understand the tendencies of the lower tables in order to get a good grasp on proper strategy. Perhaps you have found a static style that works at the 25nl level, but eventually someone might exploit that style and you'll have a hard time adjusting to it.
In other words you need to beat 5nl and aquire an adequate bankroll before moving on. Also if you get unlucky you lose your bankroll and have to start all over.
On a side note. I've never really understand how people say you need a ridiculous number of hands to prove that your a winning player. Variance is big, but no where near that big.