Commentary for Kiddushin 119:8

Rava explains that to R. Yohanan in this case the divorce is partial but death completes the severing of the ties. Therefore, even R. Yohanan can explain why she must have halitzah (she’s not fully a divorcee) but not yibbum (she is partially a divorcee).

Abaye notes that we cannot so easily combine the effects of divorce with the effects of death. Divorce causes her to be exempt from yibbum whereas death makes her liable for yibbum. Thus we need another way for R. Yohanan to read the baraita according to which she must undergo halitzah and not yibbum.