Because they were Jews, bred and born. Distrust and separation from the Gentiles had been a part of their psychological DNA for a couple of millennia.
Even Peter himself was rebuked by Paul for sitting and eating only with the Jewish Christians at their fellowship gatherings.
The Lord Jesus himself was astounded at the faith of the Roman Centurion whose ill servant He healed at a distance..................
Twice Jesus was Astounded concerning Believers;
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The Centurion and then
The unbelief of his Apostles.
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Fascinating.