It is to compensate for your auto attack wind-up. It is the longest in the game, as well as having the slowest attack speed in the game. If you have not tried her yet I recommend it as it is incredibly easy to cancel autos with her accidentally. Furthermore, in the time it takes for her auto to wind up, the entire roster of ADC's are able to close the ~50 range difference and auto her by the time hers even registers.
I'm not sure what champion you are playing where you feel her poke, as opposed to things like Brand, Lux, Zyra, Veigar, Xerath, Brand, Morgana, Fiddlesticks, Swain (need I go on) can attack from farther and do much higher burst damage, some have kill potential at level 2 where Senna's base damage is so low she simply does not.
Her mana costs are considerably high, when I play her I often go oom and the damage to mana ratio is so low it is not even close to the damage supports I usually play, not to mention her utility ability that does absolutely no damage even costs mana.
I understand people are having trouble with her W because it hits from so far but it is the typical "run straight line from a falling tree". It is one of the slowest projectiles in the game, and if you cannot dodge that (that is stopped by minions) I assume if the enemy was instead a Lux, Zyra, Morgana who all have superior, faster straight lnie roots - that you'd be rooted and killed instantly whereas Senna's weak base damage will just take a portion of your health early game.
Finally - Senna's counter is all-in champions. Play a hook champ and you get free kills for the rest of the game. Unlike the damage supports recently mentioned that may even have kill potential on you if you hook them (albeit usually die in the process), Senna can do absolutely nothing to you besides sneeze a 40 damage laser onto you. Senna is probably one of the most balanced on-release new champions I have seen from Riot in a long, long, LONG time and while she may need minor tweaks to scaling as she outscales marksman with gold, none of issues here are real.