u/MySkinsRedditAcct gave a very comprehensive answer last year. If you have access, you can read a significant discussion in Matthew D. Mendham's "Rousseau's Discarded Children" in History of European Ideas 41.1

The reason Rousseau gives is that he believed they would have a better life without him, and that his "enemies" would use his children against him.

"Never in his whole life could J. J. be a man without sentiment or an unnatural father. I may have been deceived, but it is impossible I should have lost the least of my
feelings. Were I to give my reasons, I should say too much; since they
have seduced me, they would seduce many others. I will not therefore
expose those young persons by whom I may be read to the same danger. I
will satisfy myself by observing that my error was such, that in
abandoning my children to public education for want of the means of
bringing them up myself; in destining them to become workmen and peasants,
rather than adventurers and fortune-hunters, I thought I acted like an
honest citizen, and a good father, and considered myself as a member of
the republic of Plato. Since that time the regrets of my heart have more
than once told me I was deceived; but my reason was so far from giving me
the same intimation, that I have frequently returned thanks to Heaven for
having by this means preserved them from the fate of their father, and
that by which they were threatened the moment I should have been under the
necessity of leaving them. Had I left them to Madam d’Upinay, or Madam de
Luxembourg, who, from friendship, generosity, or some other motive,
offered to take care of them in due time, would they have been more happy,
better brought up, or honester men? To this I cannot answer; but I am
certain they would have been taught to hate and perhaps betray their
parents: it is much better that they have never known them. " (The Confessions, Book VIII)

Now, Rousseau's "enemies" were often a creation of his own paranoia (witness the way in which he turned on David Hume during his period in England) and The Confessions are full of the author's protective rationalisations. The reasons given in the previous answer might provide a glimpse into the "real" motivations for doing it but his own explanation tell us a lot about him.