The French got what was provided for use by the Soviet players as all of the French pieces (excepting the infantry) are Soviet pieces in blue.
As for the Soviets in half-tracks, it’s 1) distinctive (you can tell what it’s supposed to be from across the board) and 2) it’s not the only half-track design used in the game.
In fact in the First Editions of 1940 Europe and Pacific, all of the mechanized infantry units were represented by half-tracks: Germany, Italy, and Japan used German Sd.Kfz-251; France and the Soviet Union used Soviet Zis-42; and everybody else (ANZAC, UK, and USA) used American M3 Half-tracks.
It wasn’t until the Second Editions of the 1940s games that Italians got SPA Dovunque 35 trucks, Japanese got Type 1 Ho-Ha half-tracks, ANZAC got Ram Kangaroo turretless-tanks-turned APCs, and the UK got Priest Kangaroo defanged self-propelled-artillery-turned APCs.
So, why half-tracks for mechanized infantry? Because that was what was used at the time. It wasn’t until after WW2 that armored personnel carriers became a thing.
My 2 IPCs,
-Midnight-Reaper