Crouch can't play in a 4-4-1-1, at least with van der Vaart, because he's often left extremely isolated.
When the ball comes to him he's often double marked.
This allows one player to get tight on him and the other to adjust to whatever Crouch does.
He doesn't have the pace or strength to deal with it.
If he tries to head the ball on or back, then there's always a defender there before any of our players.
If he tries to take the ball down, then he'll have one man directly behind him and one trying to get it away from him.
The defence can also play as high a line as they want, as there's no risk of a ball over the top exposing them.
When played alongside Defoe, virtually all of those weaknesses become far less glaring, as do Defoe's.
The defence doesn't want to push too far up, as they know Defoe has the pace to get in behind them off the back of any ball hit long or any flick on or pass from Crouch.
They also don't want to drop too deep, as that allows the ball played up to Crouch around the box, which can be a nightmare to deal with if he's got support, especially if it's the shoot-on-sight Defoe.
They can't concentrate too much on either man, as the other is a threat and they read each other's games well, too.
Adebayor has the pace, strength and physicality to deal with the lone role far better than anyone else that we've got now.
If you mark him tightly, then he can turn you or just force you out of the way.
This will often leave any second defender miles out of position, if he's moved ahead of the man marking him.
If you drop behind the man who's close on him, then you'll leave space between him and the Spurs midfield, which a short pass backwards or wide should easily expose.
You really don't want players like Bale, Lennon and van der Vaart moving forward onto easy balls into space, while the others move into or around the box with the striker.
If you give him space, then he might be gone by the time you've reacted.
Crouch will win most headers, but his accuracy on goal and power with them is poor, whereas the Togoan loves to attack headers from crosses.
Systems suit some players far more than others.
Some players are good wing-backs, for example, but they're not quite suited to being wingers or full-backs.