John Wesley on Numbers 19:3

Eleazar - Who was the second priest, and in some cases, the deputy of the high - priest. To him, not to Aaron, because this service made him unclean for a season, and consequently unfit for holy ministrations, whereas the high - priest was, as far as possibly he could, to be preserved from all sorts of defilement, fit for his high and holy work. Without the camp - Partly because it was reputed an unclean and accursed thing, being laden with the sins of all the people; and partly to signify that Christ should suffer without the camp, in the place where malefactors suffered.