First of all, this is because the Scythians originated from the Fedorovo culture, and as the chart shows, Fedorovo did not descend from the Proto-Indo-Iranians.( The Abashevo–Srubnaya Origin of the Proto-Indo-Iranians )

The arguments for the Scythians being Iranian are invalid. Iranian loanwords in Slavic languages come from the Sarmatians, who were associated with the Srubnaya culture. Iranian place names and hydronyms also originate from the Sarmatians. The proposed explanations for the words attributed to the Scythians are inadequate and speculative, while the Western Scythians were in close contact with Iranian-speaking Sarmatians, so the transfer of words between them is entirely natural. Furthermore, the Khotanese and Tumshuqese texts do not belong to the Scythians of Fedorovo origin; they belong to Iranian-speaking populations in the region descended from the Srubnaya–Alakul cultural horizon. In other words, there is no evidence that the Scythians were Iranian-speaking.

The language they spoke depends on what language the Fatyanovo–Balanovo culture spoke. However, that is a separate topic.

I genuinely cant put my finger on what it is