Charles Ellicott Commentary Exodus 12:10

"And ye shall let nothing of it remain until the morning; but that which remaineth of it until the morning ye shall burn with fire." — Exodus 12:10 (ASV)

You shall let nothing of it remain. — So that there might be neither profanation nor superstitious use of what was left. (Compare the requirement of the Church of England with respect to the Eucharistic elements.)

That which remains — i.e., the bones and such particles of flesh as necessarily adhered to them. These were to be at once totally consumed by fire. Thus only could they be, as it were, annihilated, and so secured from profanation.