I guess, because Quag is the most versatile. He is the perfect 3rd CF/SS/RWF/LWF. Besides, he gives you a different dimension to his game as it is very unique. The only problem is- he will want game time and at his age we would have to give him at least another full season before he accepts the bench role... if ever.
Cavani, Quag, Borriello - would be good enough imho. In case we are offered Borriello for pennies, he takes a huge salary cut and we can fetch 15m at the very least for Matatatri. Obviously, Demba Ba instead of Borriello would add a lot, but in all honesty I don't see it happening unfortunately.
I am currently in dismay about us going after both Suarez and a CF(Cavani, Higuain, Falcao?). Suarez would change everything and if he comes I guess it will be better if Quag leaves. Suarez would take Quag's game time completely and, apart from sick long shots, he has so much more in his arsenal and is even more versatile.
In case we get the Uruguayan our attack should be: Top CF, Suarez, Borriello/Ba/Quag (cheapest replacement), Youngsters... and Suarez would be usually deployed as RWF I guess, or SS if we are playing with 2 in front.