There's little to no charging infrastructure built out at their depots and having to implement that rapidly would as substantial cost. MUCH if the USA that the USPS services is ex-urban and rural where fleets in those areas are smaller and distances covered may actually require more than a single daily charge. The vehicles are designed for EXTREMELY long service lives, so full battery change outs probably 2-3 times per vehicle would need to be amortized into cost of operating. Auto-stop tech on ICE vehicles is likely built into these, thus reducing much of the idling expense, and I'm sure there's more.
Politics surely played a role but the USA, just in sheer land area too service for a national service like the post office is of a scale no euro country can accurately compare to. Austria (which I saw in another comment) alone is the size of ONE mid size state. I suspect they'll scale up depot charging capacity and test viability on the 10% they're converting but going all-in is quite plausibly not viable.
Similar to how for MANY people EVs could work great as their only car, but many others they don't make sense (no at home charging, cold climates, lack of local charging infrastructure, long distance driving, etc...).
EV enthusiasts seem to tend to be upper income suburban/urban lifestyle folks, but there's a blindness to the rest of the country where that isn't there income bracket, driving style, or climate they live in but are fine forcing 100% EV on others who literally can't afford it, or makes their lives notably more difficult to move around with.
When I can get a decent used EV for 20k or less that'll last another 100k+ miles (as my last 3 used cars have, which I park on the street) then it'll be for me, right now it isn't, and I'm not alone.
The USPS operates across a similarly broad range if conditions.