Because Viana's a centre midfielder and at the time was rated as one of the hottest prospects of continental football.
He won European Young Player of the Year ,the season before he joined us and we beat alot of big clubs to his siganture I seem to remember, such as Juventus at the time. I think his game is suited to the slow pace of Italian football rather than Spanish and especially the EPL.
And as already mentioned, he was never supposed to compete with Robert for the left wing spot. He was not a left winger in the slightest even though we shunted him out there. His best position was in the hole behind the striker(s) with a proper defensive midfieler doing all the donkey work (which we didnt have) ......and most EPL teams including ours don't play with that formation
The main thing that fucks me off about the Viana transfer is that in the games we would've watched him (Portugal u21's and Sporting) Ronaldo would've been playing as well and for the same fee (ish) we could've had him.