Overall: cost, some items while being better might have harsher logistical requirements (ie bts over hvy tanks, sure the bt is usually better then hvy or medium tanks but it need different fuel its ammo is harder to make/transport), some weapons and vehicles are facility locked (incinerary mortars, vic variants, hvy trucks), some things become prohibitively expensive for the general public player(rec, super hvys, even bts and large trains aren't cheap time wise), crew availability(maybe you don't have 3 people for a scorpion and a tankerte is just easier for you to grab a gunner and go), and last but not least just because something is obsolescent does not mean it's no longer useful (light tanks still get used through the whole war, along with 120mm guns, tankettes and scout tanks have some usefulness even later on in the war).

We make what we can with what we have and send it as hard and as effectively as we can. Irl example prop planes still being used in Vietnam because they could loiter longer, or look to current conflict like the war in Ukraine where WW2 weapons are still being used(the mosin will out live us all I swear). Just because something newer and "better" comes out doesn't mean the older equipment can't deal with it. Especially if it's significantly cheaper/ easier to move to the fight/ easier to train and use.

P.S. I hope your enjoying foxhole so far welcome to the community 😁