Because top strikers want champions league footy. At their pay, cash is not everything, almost, but not quite.
That is limiting the amount of strikers we should be going after though. I mean just to name a few
Falcao (Porto + athletico)
Manduzic (spell?)
Bacca (sevilla)
Dzeko (city)
Benteke (villa)
Aguero (city)
Lebondowski (Dortmund)
Off the top of my head. None of them moved for champions league in their career before they became top stars. They were All signed up before they were household names and got their big money moves afterwards or got that champions league at their then clubs. Point is, there are many strikers out there like the above that money would buy now , they may not be the finished article but at least they are a blue before they become it and we benefit from the ability and goals.
There are thousands and thousands of strikers out there to buy. Surely we can go and buy one that isn't a world renowned name but will still be a good signing? That's what our dof and scouts are for after all and they don't seem to venture very far south of England never mind abroad