Pilot Skills Ep.6 - Mana abilities vs DRS vs Kinnan #cedh #mtg #commander #magicthegathering 

Because, with the previous text, you could announce that you were casting a card from your hand, put it on the stack, then activate LED and discard the rest of your hand to pay for it. This effectively made LED almost as powerful as Black Lotus. Originally, you needed to have the mana already in the pool before you could cast anything, but now that you don't, LED's original text allowed such an exploit, thus the need to add this restriction so that LED kept its intended function.