It may not be Stevie Wonder holding up the proceedings, but the 1972 Copyright Act that went into effect on February 15, 1972. It stipulates that music physically recorded before that date can be used without permission from the copyright holder....Music physically recorded on or after that date requires the permission of the copyright holder before it can be used by another party. This is a very basic summary of the Act. The link below has all the head-spinning details!

Considering that in 1972, Motown was licensing a lot of outside artists for release, plus the diversity and relatively large numbers of artists directly signed, the individual separation agreements for some may have given the artists copyrights to their masters. It is probably a logistic nightmare trying to track down the current copyright holders and get their permission for many of the tracks...

I'll bet HW wished they had stopped at 1971!!

Music copyright info.....
http://wapedia.mobi/en/Wikipedia:Public_domain?t=8.