Why are gram-negative bacteria are more resistant to biocides than gram-positive bacteria??

The main factor to the resistance of gram-negative bacteria to biocides is the external lipopolysaccharide layer. Their porins (the structural openings in the wall of gram-negative bacteria) are very selective of molecules they permit to enter the cell. Therefore they resist the entry of antimicrobials and certain antibiotics.

Why are gram-negative bacteria are more resistant to biocides than gram-positive bacteria??
Biocides are designed to attack gram positive bacteria - which would explain why they are more vulnerable to their attack.