It's the fact that most plenums stick out the bonnet. Side draught carbs fit better space wise. 
 
Also a lot of engine solutions are using engines where the stock management is entangled with the security system to such an extent that the car won't run without a lot of work.  
 
Ideal world is a set of throttle bodies on 3rd party management. 
 
TT 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
	 |