War Thunder: Jagdpanzer 38(t) Hetzer German Tank Destroyer Gameplay [1440p 60FPS] 

Actually Czechoslovakian door wedge

Long gun, no space inside.

tank destroyer is made to destroy tanks soo they have to have super cannons and good mobility the armour is an option

Theres many types of TD variants in ww2, but most commonly is the famous two, light and heavy TD (referring to armor) and this case for this tank its heavy because most of the armor is on the front, so you engage enemies with your hull aimed at the enemy so a rotating turret isnt required, and so reducing the overall armor for better mobility or bigger gun, for example this TD in the video can be killed easily from any side especially the roof, but have good mobility and small figure The other famous type is the light TD for example the M18 hellcat, big gun good mobility no armor Then theres post-war TD like M50 Ontos, M56 Scorpion, T92 etc and all of them prioritize the mobility and no armor since many nations thought that armor was considered useless after the invention of chemical rounds and ATGM

In summery if you create a Tank destroyer and want a rotating turret you will end up with something really slow or really fast without armor