Very interesting comments regarding the Beggars Opera first LP. I was lucky enough to see the group 4 times in the period 71 - 73 (I lived in Scotland, near Edinburgh at the time) and I was amazed that they did not receive the recognition they deserved. They were great live and their first 3 LPs were a joy to listen to.
Their 4th LP (Get You Dog Off Me) was somewhat different in style - the singer had changed. However, I enjoyed Dog - but it was obvious that the group had changed their style in an attempt, I think, to make it "bigger" and, perhaps, to be noticed in the American market - were the big money was..
The sound of the organ and mellotron was a joy to hear - Act One containing some absolutely outstanding Hammond B 3 work from Alan Park.
Re the question Why did they not make it to the "big time"? Remember, at that time (very early 70's) there was just so much music around - new groups coming and going/members leaving to form other groups/etc. In addition the "majors" at the time (e.g. D Purple/J Tull/P Floyd/LZ/U Heep/Yes/etc) just became bigger and bigger, and attracted all the attention and sales. Thus, I think, the music of Beggars Opera was lost in mist of an exceptional period of time for great music. As mentioned above, their style changed with the 4th LP (Dog) in 1973 but it failed to attract the attention of the LP buying masses of 16-20 year olds and then they were gone. However, as an example of the standard of music produced, let us remember that Beggars Opera produced 4 great LP's during the period 1970 - 1973: an outstanding achievement.
Incidentally, my copy of Act One includes their first 45 single - Sarabande - which appears to sound like it is an extract from Passacaglia - brilliant wah-wah effects on the guitar and Hammond.