Probably all the usual reasons Roburt.
As you know , companies like ABC, and Motown to name another obvious one, had release schedules planned well in advance but often never stuck to those plans in terms of release dates or even releasing the records at all .
Other considerations like releasing product earlier or later than planned if the artist, for example, was touring the country in order to gain maximum exposure for the record etc etc.
Interesting you specifically mention the Blue Notes though. In their case, the big split around that time (1976) between Harold Melvin and Teddy Pendergrass and then the Blue Notes themselves splintering into two groups (ABC & Glades) with all the possible legal ramifications may well have had something to do with the release timing of their first ABC LP.