Because, according to Muslims, Muhammad was the better man.
Why? The idols (according to Islam) had no power in and of themselves. The idols weren't the ones torturing or persecuting Muhammad for preaching Islam. If they had no power to begin with, moving the idols out of the Kaaba would have had exactly the same effect as smashing them : nothing.
Okay, first off, that's not what I asked. I asked how you think making Muhammad out to be the better man that Islam claims him to be (i.e. a moral exemplar that Muslims should emulate) would weaken Islam. The direct implication of what you're saying is that making Muhammad appear morally upright and tolerant of other beliefs (where none was offered to him) weakens Islam - thus implying that Islam is at its strongest or purest when it is at its most intolerant.
Second, who is this "they" you're referring to in possession of the "left overs" of Islam (whatever those are)?