Because it's about the quality of our team right now, and our ability to challenge with the players we have. Lamela looks to be a great signing, but he isn't Bale. He isn't going to score 30 goals. If he was, Madrid would have gone for him. Even though it's a good signing, if it was our only signing it would be a downgrade.
We have strengthened elsewhere. We have a serious striker now. Paulinho and Capoue are massive upgrades on Parker and Huddlestone. Chadli is a very nice addition. But I would say to truly move forward even when losing Bale, we need at least 2 quality players to come in for the attacking positions. Lamela would be one, but because he isn't on Bale's level yet, Willian is the other. If Chelsea get him and we don't, it boosts the quality of their squad and keeps us down. Just like if Liverpool got Willian, they would be very serious challengers to the top 4 spots, and we needed to stop them. I'd even say if Liverpool got Sig and Dempsey last year they would have been a lot closer to us and helped Rodgers' system fire sooner.
So we have made upgrades. But so have the other clubs. To catch up to them we need to go above and beyond what they are doing, and Willian would have achieved that. If Chelsea get him, and Arsenal get say Di Maria and Benzema, we are not looking as strong. My only hope is that if Chelsea get Willian, their marginal improvement is so small in terms of pulling them ahead of us because he won't get as many games. But it just means that if they have any injuries or drops in form, they are ridiculously well covered.