Vayne Tumble - Why so low CD and Mana cost

I was playing jungler at level 14+. was opponent's adc. She was alone.

1. I started E + Q jump on vayne. Did some damage.
2. As soon as she was out of stun, she tumbled. - Wiki says she'll be invisible during the tumble.. i.e. nothing can target and obviously my 2nd casted E had no effect .
3. She did 3-4 tumbles draining all my health. I can't Q or E her .

I later looked up to check how can she tumble so quickly. Apparently her CD is 2 sec. With some CDR .. she is invisible, untargetable. If she is lvl 14 (like me ) she can probably tumble 25 times.

I have played against vayne many times, without a hard CC she is tough to kill in late game. So my question is why is her ability has so low CD and low mana requirement? Considering she is an adc/marksman who does damage using AA not ability.