Why are old LEGO bricks more brittle than new ones?
The LEGO company switched to ABS in 1963, bricks produced before that have a markedly different quality.

However, if your sets are from around 1980, they should use ABS already and there should be no reason that the plastic is different.

Other possible explanations would be bad storage conditions (too warm, direct sunlight, maybe), or the fat that the set may have only been built once. Indeed, ABS bricks reach their optimal clutch power after having been assembled/disassembled about 8 to 10 times, so if you just built the set once and let it be (for display for example), then the bricks are in fact still too new and clutch together too tighly.