We, at Sunderland, were told by the Bridgett, Parker Promoters back in 1964 that poor weather and poor crowds were the reason for closure.
None of the nine Meetings they ran before closing down was rained off, and the crowd at the penultimate Meeting was 8,500 (Source: Sunderland Echo). The reasons for closure that I believe to be true, can be seen in the Link provided:
http://www.sunderlandspeedway.co.uk/stats/1964/1964_Season.html