Simple. Because startisback is made better and has more capability. I realize that isn't for everyone, and by all means try them all to see what you like. Classic shell is good too, I just prefer startisback.
In my line of work, its easier to sell someone (referring to a business that I would do service work for) on a software that has professional options (such as volume licensing) and is more configurable, rather than a free program. Lots of businesses don't see something you don't have to pay for as being worth using and will often times simply write free stuff off without looking into them. They can't see past the "free" stereotype. Speaking from experience. We're not talking about folks that care enough to learn anything about what they're using or could use, tech-wise. They just want to pay someone to make it work as simply as possible, and if you tell them "oh yeah here's this free thing" then they might write you off too lol.
But yeah, there are a ton of programs that fall into this category. Classic shell would probably fall in as #2 in my mind.