There are a few reasons. The biggest one is that 18% AS only provides a marginal increase in damage that can be made up for in itemization. Instead taking lifesteal in runes opens up itemization later in the game. Tenacity frees up boots to allow for more diverse options, and frees up runes to allow for Overgrowth as needed. Ultimately there is no one correct runepage, so if you find success with alacrity, go for it. It comes online much much faster, but has less impact than the other two can have.