Because prior to the 20117/18 season we constantly mixed the 10/12 line ups eg: Williams/Tait, Mermoz/Roberts, Roberts/Betham, Williams/Roberts, Mermoz/Smith etc etc. Our pattern/style of play was as inconsistent as the selections. Toomua's world class experience imparted into the backline and sure enough, we started the 2017/18 season consistently well, but more importantly we had options in attack, a plan B when things went tits up!! and also a better affiliation amongst the players. Just because Toomua got injured, it didn't stop his input on the training ground, and that possibly was the continuity that has helped our backline's resurgence in recent times.
All this is just an opinion Raging Bull, but I can remember very well sitting in the stands watching a clueless backline (full of talent!) for virtually an entire season, play the sort of rugby that would possibly beat a lot of sides, but not have a cat in hells chance of beating the teams that matter ie carries and Wasps et all.
Now that we finally have a punching and dynamic back row, the Youngs/Ford/Toomua combination i'm sure will put people like Manu through the gaps instead of just smashing into brick walls, and also give our excellent wings/FB space to carve up.