Because if you do, your competitors won't, and will poach your customers with hard sell techniques until they reach a network effect required to make you irrelevant.

In the long run, there can be only one, the little diversity we have is the effect of national network effects established in the first years of growth that proved hard to overcome, especially Twitter in US. Without US, Twitter will be steamrolled and you will only have exotic alternatives like vkontakte and weibo.