Two principal reasons:-
1/ Mintages
1961 - 48,313,400
1962 - 148,308,600
1963 - 125,235,600
1964 - 153, 294,000
1965 - 121,310,400
1966 - 165,739,200
1967 - 654,564,000
2/ I bought some old coin monthly mags from the late 1960's and early 1970's, off Rob a couple or so years back, and the late 60's was very much a boom time for the coin market. It was obvious from looking at some of the dealer ads, that dealers had bought many mint rolls of mainly later into the 60's pennies, such as 1967, and were offering them for sale to the public.
I reckon a combination of the two points above is pretty much why 1962 to 1967, is cheaper than the 1961. There are fewer 1961 pennies available in BU, than the other years, especially 1967 itself.