Part of the reason is typically later in the summer we have a higher likelihood for stronger llvl moisture which; 1) leads to lower LCL’s 2) enhances llvl instability and 3) combine this with strong shear in the 0-3km layer and you have potential for weak tornadoes. Basically we’re relying more on what’s going on in the llvls than the mid/upper llvls...especially when we aren’t really looking for storm tops exceeding 20,000-25,000’. I also believe with warmer SST’s this reduces the effects of marine air, especially in situations where dews are exceptionally high.
Also, steep mid-level lapse rates are typically more important when looking for significant severe widespread severe weather...this includes strong tornado potential.