There is a non-supersymmetric heterotic string theory with O(16) x O(16) gauge group. This theory has been known since the eighties - read "An O(16) x O(16) Heterotic String" by Alvarez-Gaume, Ginsparg, Moore and Vafa (https://inspirehep.net/literature/22725) and "String Theories in Ten-Dimensions Without Space-Time Supersymmetry" by Dixon and Harvey (https://inspirehep.net/literature/227374).
I think one reason this theory is not too well-known is just that (compared to superstring theories) it is harder to compute anything. I myself don't know much about this theory, so I don't know for sure, but I don't know of any simple way in which one can build semirealistic models from the O(16) x O(16) theory as a hobby. Furthermore, supersymmetry, while not yet directly observed in nature, does have some phenomenologically nice properties, so there are good reasons not to throw it away.
However, Vafa seems to have done some more work on this theory, and there seem to be other people working on it too.
There is also type 0 string theory, which however doesn't contain fermions and doesn't really count as a realistic theory.