Albert Barnes Commentary Mark 6:8-11

"and he charged them that they should take nothing for [their] journey, save a staff only; no bread, no wallet, now money in their purse; but [to go] shod with sandals: and, [said he], put not on two coats. And he said unto them, Wheresoever ye enter into a house, there abide till ye depart thence. And whatsoever place shall not receive you, and they hear you not, as ye go forth thence, shake off the dust that is under your feet for a testimony unto them." — Mark 6:8-11 (ASV)

In Matthew 10:5, they were commanded not to go among the Gentiles or Samaritans. Mark omits that direction, perhaps, because he was writing for the Gentiles, and the direction might create unnecessary difficulty or offense. Perhaps he omits it also because the command was given for a temporary purpose, and was not in force at the time of his writing.