For people who already own the Vocaloid 4 editor, there's no advantage to buying an entirely new editor, especially when you lose V4-specific features like GWL and XSY. The upgrade cost was also prohibitively expensive at initial release due to the editor being bundled with multiple voicebanks.
At around the same time of V5 and V6, SynthV gained popularity as a serious competitor. Many beloved vocaloid characters got updates on SynthV. SynthV is also more affordable, higher quality, and easier to use.