Kevin
I'm no expert but there was (like most things) probably many reasons why it was this way.
a) they probably were strongly opposed to anything remotely like episcopalian, or high church practice, probably assuming weekly communion for instance would lead to on one hand superstition or on the other hand casualness. (there is some merit in both concerns, but I don't believe the solutions accepted helped!)
b) there was frequently a great shortage of ordained ministers (although I said 2x a year I think 4x a year was more common in urban areas, where there was a greater population of ministers close by.
c) It should be noted however that each communion 'season' may have lasted between 3 days to a week! So for instance there would be meetings on the Friday, Saturday, Sabbath and Monday, in preparation for and appropriate conclusion to the sacrament. (BTW this shows that these were serious-minded and spiritual folks.)
The downside of all this was that many feared to publicly confess Christ by coming to the LS, such great importance and solemenity was given to these seasons that people were literally afraid!
The upside was that not infrequently revivals small and large broke out in and around communion seasons, e.g. the Cambuslang Revival in S.E. Glasgow.