There can be two answers. One is why it "really" happened - namely, what stood behind the decisions of Stalin and Yagoda (e.g., social conservatism, personal homophobia...). On this one can only speculate. It can only be said that from the start the Bolsheviks in general were certainly no friends of gays, though they could tolerate them at first. In the 1930s the likes of Koltsov and Gorky tried to connect homosexuality to fascism (incl. Nazism). Acc. to Gorky there used to be a sarcastic saying: "Destroy the homosexuals and fascism will disappear" (he seemed to agree with it).
But one could also answer what sequence of events in particular led to the law in question, whatever the "real" reasons. And this was the pogrom of the alleged gay "brothels" by OGPU. A whole "case of the Leningrad homosexuals" arose from these and other raids, with about 150 men arrested.
Yagoda ascribed counter-revolutionary activity to these gay circles:
https://www.gazeta.ru/science/2018/12/17_a_12097333.shtml
https://a-g-popov.livejournal.com/1422055.html
"The active front of the homosexuals, by using the caste-like isolation of homosexual circles for directly counter-revolutionary purposes, politically decomposed various social strata of the youth, in particular the working youth, and also tried to penetrate the army and navy". (Here and everywere only male homosexuals are meant.) "... homosexuals were engaged in the recruitment and corruption of perfectly healthy youth, Red Army soldiers, Red Navy and individual university students..." Yagoda also pointed out that there is no law against male homosexuality, and proposed to introduce one.
Stalin's resolution was: "It is necessary to exemplarily punish the bastards, and to introduce the corresponding governing decree into the legislation".
Molotov: "Of course this is necessary". Kaganovich: "Right!".
Soon thereafter the infamous law appeared.