Because T-34 ram. Axis stunning abilities (only Target Weakpoint and variants) are defensively orientated.
There will be clearly enough opportunities in every game where allied tanks are in range of you PAK. Puma, while beeing highly mobile, can stun a nonturreted tank (-> SU-85) at decent range without sacrifing itself (like T34). StuG E, not as mobile as Puma but mobile, has the option to stun a tank too.
You can call in the JU-87 at that opportunities.
You can't charge into a (heavy) tank and call down the Stuka strafe with them, unlike with a T-34 and IL-2 rockets.
Its not that indestructible T34 hordes charge frontally into axis line. There is always the risk to loose your T34 without having achieved anything. You have to wait for opportunities too, but they are different from the axis ones. The outcome is the same: Calling an AT-strafe at a hampered tank. Btw. I thought ram was nerfed for that reason (having a chance to get the tank out of the strafing zone).
Combined with lower cost, it'd be okay for the IL-2 rockets to be less effective.
Did I missed something? IL-2 rocket strafe cost 15 MU more than JU-87 AT-Strafe.
If you go for a ram you are investing an additonal 300 MP and 90 FU.