Because they're only aiming at high tier, not necessarily hitting it. There will inevitably be some variation, especially when the top pros get their hands on the new characters' new tools and develop the metagame, using things in unforseen ways and revealing character's hidden abilities. Some characters will turn out to be around Melee mid tier, others at top, and even possibly above the Spacies.
And if we want this game to succeed, we will need the support of great and famous players, IE Spacie mains. They're not necessarily purists, and can be persuadable, but we'll have to offer them something they'll accept, and almost no one ever accepts nerfs to their main. (BTW, haven't seen too much TL in streams, so just going on hearsay, but if it doesn't already, his Up-B needs to at least go as far as Melee YL's)