The key difference between the progenitors for the three types of core-collapse supernovae is the presence or absence of the outer hydrogen and helium shells before the explosion. SNII retain both their hydrogen and helium shells, resulting in the clear detection of hydrogen emission in their spectra. The almost featureless SNIc fall at the other end of the line, and appear to have lost both their hydrogen and helium envelopes through some mechanism prior to the explosion. In the middle we have SNIb, which have retained their helium envelope but appear to have lost their hydrogen envelope prior to the explosion. For this reason we do not observe hydrogen lines in the spectra of these objects but we do observe helium lines.
Exactly how the hydrogen and helium envelopes are lost remains unclear, however, it could be due to mass transfer in a close binary system, through a common envelope phase, strong stellar winds or a combination of all these possibilities. That some of the mass of the progenitor star is lost before the core-collapse is not in doubt since SNIb and SNIc are detected at radio wavelengths indicating that they are surrounded by circumstellar material.