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

They're not. There are highly antibiotic reisitant gram positive bacteria such as MRSA and vre and even rarely vrsa among others. There are also multidrug resistant gram negative bacteria resistant to almost every antibiotic known to man such as some strains of pseudomonas aeruginosa, acinetobacter baumanii and metalo-carbepenemase producing klebsiella.