Because 99 out of 100 folks who try to do more than one thing fail.
(For those who do not know the relevance of this picture in this post, click here to read the story of Arjun and the fish eye).
Focus helps startups concentrate their efforts and resources on doing one thing well. Even if that does not work, because of the focus and attention, they are able to smell the failure earlier, and can then plan the pivot and refocus.
If you do more than one thing, your resources and bandwidth is shared. Your efforts do not carry the same weight that a focused effort can. And, in the midst of the chaos you tend to ignore (or miss) the signals that things are not going as you intended them to.