Joseph explains why plates were returned to angel: mobs tried to steal them.
History, 1838–1856, volume A-1 [23 December 1805–30 August 1834]: 8, The Joseph Smith Papers website, accessed August 11, 2021
I soon found out the reason why I had received such strict charges to keep them safe and why it was that the messenger had said that when I had done what was required at my hand, he would call for them, for no sooner was it known that I had them than the most strenious exertions were used to get them from me. Every stratagem that could be resorted invented was resorted to for that purpose. The persecution became more bitter and severe than before, and multitudes were on the alert continualy to get them from me if possible but by the wisdom of God they remained safe in my hands untill I had accomplished by them what was required at my hand, when according to arrangement the messenger called for them, I delivered them up to him and he has them in his charge untill this day, being the Second day of May, One thousand Eight hundred and thirty eight.