I think that, also, Decca had lost an awful lot of traction when it came to chart success by 1974. I have Vol.7 (1973), and it contains quite a lot of tracks from earlier times - three from the 60s (Dave Berry, Billie Davis, Bowie) and one from the 50s (The Goons). Of the others, the Bloodstone track - as good as it is - only just made the Top 40. So, even if the World Of... series came to an end shortly afterwards as PhilMH suggests, then they probably wouldn't have had the material for a Vol. 8 without stretching the concept to breaking point.