Try talking to more investors or bootstrapping.
Partially, it's a business continuity issue. The other part of it is that startups with multiple founders tend to grow faster as the founders feed off each other.
If you're the sole founder, and you die. It's over. Nobody else is going to take over with the same energy, passion, and insight you had.
Many accelerators and investors have a basic position that bore out in the research undertaken in 'the startup genome report'.
'..the findings confirm what are commonly held beliefs, such as the drawbacks of being a single person founder. Solo founders take 3.6 times as long to reach scale stage as do founding teams of 2 and they are 2.3 times less likely to pivot.' http://readwrite.com/2011/05/28/the-startup-genome-project-e...