This is a wrong question. It should be asked why 1C has electron charges. The electron charge is both a fundamental constant, and an elementary value.
A charge magnitude lower than an electron can’t be observed. Non-interference charges can be found in quarks, but they cannot be seen separately.
On the other hand, the Coulomb is the charge carried by one Ampere per second. These values are constructed and do not have the same fundamental meaning as an electron charge.