You can compare the margings
Localbitcoins will always be better  most times than a start ups, also the selling rates are much better,plus there is instant contact and trades using IMPS payments unlike these start ups with delays where you can lose precious trades and times until your order is fulfilled

To answer your question more specifically:-
Lcalbitcoins has been operational from a long time than others start ups so you can assume most purchased bitcoins at cheaper rate years ago  so even if the sell at marginally lower rates now,they still make a fortune considering the rise in value of bitcoins today
Plus P2P trades generally costs lower since you are not dealing with a company which pays various fees,taxes etc that contribute to increased costs