I've noticed that some thriving worker cooperatives were started by individual entrepreneurs who built the business then spread ownership. Just because workers can do the work in the daily operations of a business does not mean they can start, build, and market the venture. Some cooperatives start by "borrowing" the leadership in the form of non profit techical assistance where the cost of leadership is paid for by a third party.  In private enterprises without such outside help, the leadership or technical expertise expects (not unreasonably) to have their contribution rewarded.

Another reason is start up capital... as I understand it, the Cleveland experiment was dependent on foundations and other charitable and government sources for most of start up capital.  When start up capital comes from a private source, it expects a return on the capital or at least their money back... even people who lend through Kiva want their money back to reloan again!

Another thing I've noticed is that sometimes people working in coops  want to be given credit for "equity share" when they spend time on non business related activities... like protesting one thing or another. Too much of that means people want to be paid for doing nothing that will sustain the business.  Too much of that attitude may hinder the long term viability of the business.