Because a correlation doesn't prove theres a link. To prove there is a link you need to prove causation, you need something which says that if X happens there will be a change in Y because of Z.
You have numerous factors which have changed alongside the taxation rate. You have changes in tax law, changes in the quantity of income and wealth and you have changes in the rate of taxation. You have any number of things which have changed and therefore the change in total tax income from that bracket vs the changes in overall taxation of that bracket are useless when taken alone. You'd have a strong case if the only changes which had been made were the level of taxation and the quantity of revenue from taxation from that economic group.
With your example the only information which was applicable would be the number of rainy days vs the number of pop tarts sold. Its a pretty simple equation and its easy to imply causation between the weather and number of pop tarts sold because there are so few variables outside of those two variables which could effect the equilibrium.
Tease.
