Jp4 while its own beast isent used cuz the panther overlaps and is a safer choice.
Jp4 and Stug are med TDs, they are not ment to face heavy targets. And as for AT mesures against meds, you either can use your own meds or AT guns. When it comes to facing anything else, panther becames better chose, because you can dive in and take a lot of punishment.
You never will be able to out-spam allies TDs with JP4, aswell as, when properly played, allied TDs have an upper hand over stugs.
Allies TDs on the other hand can effectively fight any kind of armored unit, equally effective and equally forgiving.
People mostly go for panthers, simply because they are reliable, you know if you are investing in them you wont be fucked up by rng that bad.