What are the evolutionary reasons behind men being taller than women?
I assume by asking why, you are asking about the distal evolutionary causes, and not the molecular mechanisms that account for these things.

(Important disclaimer: these causes are difficult to be certain about; they require a fair amount of informed speculation.)

With that said: it is widely agreed upon in evolutionary biology that human males, as in other mammals, are thought to be larger mostly due to sexual selection. In other words, size is sexy, and sexy individuals fare better with respect to mate choice, on average. Sexual selection can also work via male-male competition. There may be other causes that are relatively minor contributors compared to this one.

Notice, however, that in humans, females evidently do not remain tiny with respect to males, but they get 'dragged along' in terms of their height, so that males are not drastically different in height (unlike, for instance, in elephant seals). This too may be a result of sexual selection, quirks of physiology across the sexes (e.g. genetics of dimorphism), or even natural selection. Think hip size and the size of human skulls.

Additionally, differences in development time (e.g. length and onset of puberty) can skew body size. Humans certainly exhibit dimorphism with respect to development.