Deuteronomy 24:4

Her former husband which sent her away may not take her again
to be his wife
Though ever so desirous of it, and having heartily repented that he had put her away: this is the punishment of his fickleness and inconstancy, and was ordered to make men cautious how they put away their wives; since when they had so done, and they had been married to another, they could not enjoy them again even on the death of the second husband; yea, though she was only espoused to him, and he had never lain with her, as Ben Melech observes, it was forbidden the former husband to marry her; though if she had only played the whore, according to the same writer, and others ^F1, she might return to him:

after that she is defiled;
not by whoredom, for in that case she was not forbidden, as it is interpreted, but by her being married to another man; when she was defiled, not by him, or with respect to him, nor with regard to any other man, whom she might lawfully marry after the decease of her latter husband; but with respect to her first husband, being by her divorce from him, and by her marriage to another, entirely alienated and separated from him, and so prohibited to him; and thus R. Joseph Kimchi interprets this defilement of prohibition, things prohibited being reckoned unclean, or not lawful to be used:

for that [is] abomination before the Lord;
for a man to take his wife again, after she had been divorced by him, and married to another man; and yet, such is the grace and goodness of God to his backsliding people, that he receives them when they return unto him their first husband, and forsake other lovers, ( Jeremiah 3:1 ) ( Hosea 2:7 Hosea 2:19 ) ;

and thou shalt not cause the land to sin which the Lord thy God giveth
thee [for] an inheritance;
since if this was allowed, that men might put away their wives, and take them again at pleasure, and change them as often as they thought fit, no order could be observed, and the utmost confusion in families introduced, and lewdness encouraged, and which would subject the land and the inhabitants of it to many evils and calamities, as the just punishment thereof.