The answer is technological limitations. And it has got nothing to do with spark plugs. Rather it has been aim of engineers to increase the compression ratio to higher values. But the trouble is the phenomenon of knocking.
Here is how it works, as the piston compresses the air fuel mixture in the cylinder and reaches TDC, spark plug fires, igniting the mixture. This air fuel mixture is essentially homogeneous in nature for gasoline engines (heterogeneous versions in form of stratified charged engines have also been developed). The mixture burns with flame and this flame travels from spark plug side to other side of the combustion chamber and forces the piston down due to increased pressure. This flame is meant to consume the unburnt mixture while traveling. But, it does not always happen. Engine contains various hotspots (regions with high temperature). This results in self detonation of air fuel mixture at those points, with their own flame propagating. As these two flame collide each other, a boom noise is released with lots of energy which can cause damage to engine.
This phenomenon of knocking increases depends on fuel properties and increases with increasing compression ratio.
Thus, comes the octane rating of fuel. On a scale of 0 to 100, higher the number better is fuel to prevent knocking. That's why you have special fuels such as Speed in India which have better octane number and are costly too. Earlier, lead was used to be added in gasoline as it used to enhance octane number. But slowly its poisonous effects were discovered and it was done away with. Thats why you hear unleaded petrol these days when you visit a petrol pump.
Now, a lot of effort has been going to increase the compression ratio without increasing the knocking tendency. So far researcher have managed C.R of 12. Hopefully, we will be able to push it further.