A 1968 factory 10.5:1 engine burned 100 octane gasoline, with tetraethyllead {[ch3ch2]4bp} octane booster[Lead}, Ethyl. In 2015 gasoline is 87-93 octane with ethanol [crap in my opinion]. Your 1968 10.5:1 engine could run trouble free in 1968 as it was burning gasoline with an addative which inreased the octane level of the fuel and in turn one could increase the compression ratio considerably over fuel [gasoline] without lead. We no longer have "Ethyl" which is , to answer your question, why builders are reducing compression ratios from 1968 standards.
Then there's the static and dynamic compression ratio comparison. No mater what the compression ratio , it's the cylinder pressure we are concerned with. The higher the cylinder pressure the more power and the more chance of detonation. Static compression ratio is the ratio of the volume of the cylinder and combustion chamber from bottom dead center of the piston to the volume of the cylinder above the piston and combustion chamber at top dead center. [swept volume] Dynamic compression is the volume of the cylinder when the intake valve closes compared to that at top dead center. One could have a high static compression ratio say 12:1 with a lower dynamic compression ratio than that of a 10 .5:1 static engine ,depending on the camshaft. Dynamic compression ratio will be determind by your camshaft. Take an engine with a compression ratio of X and change the opening and closing points of the valves and the compression ratio will be +or - that of the original.