Because the 340 took the thunder away from the 318 as the only performance engine for small blocks. MoPar/Chrysler never got behind the 318 as a engine that could perform. It was a basic people mover engine. Even though it has many advantages.
Getting a 318 into the mid 12's isn't to hard, just so much easier and street freindly with a larger engine. Until recently, no one really made a performance piston for a 318. KB makes a few that are resonable in cost, but still more expensive than the larger engines.
On the cheap, after a bottom end rebuild with high compresion pistons, it's just a big cam, gear, converter away with 360 heads and a capable intake and 750 carb with headers.
Engine parts for this could be done under/at $3000 after the shortblock is rebuilt.
Gear, converter, tires, suspension and chassis work extra.