No idea why. No it isn't, hence why they stopped making side feeds around 2k1. Collectors might find certain side feeds more valuable if thats what they are looking for but I believe most ppl like their cockers to be VF.
Side feed is better b/c you can look down the sight rail for a change without a huge hopper blocking the view. Also, you don't have to hold the Autococker at a weird angle like you do with a center feed one.