Why do spectroscopic binaries have approximate circular orbits?
Spectroscopic binaries are not necessarily close and not necessarily in circular orbits. If the orbit is circular, then a time series of radial velocity measurements for one or both stars would be well modelled with a simple sine wave.

However, eclipsing binaries are almost certainly close, since eclipses in a system with widely separated components are improbable (but not impossible). If you have the eclipsing light curve, then the primary and secondary eclipses will be equi-spaced in time if the orbit is circular.

The key factor for circularisation is the orbital period. If it is less than about 7 days (for solar-type stars), then circularisation due to tidal forces is almost certain.