Because more than a dozen Panzers (11 IIIs, 2 IVs, and 2 PaK 36 anti-tank guns) were destroyed by
a single Char-B1 in an ambush (they ambushed it), and the Char was still drivable afterwards. That was 16 May 1940 at Stonne.
And of course Arras, where Matilda II's wreaked havoc on the SS and were only stopped by 88's brought up by Rommel...
Sure, these were nasty shocks but hardly sustained defeats for the German infantry. It wasn't a secret to the German High Command that their tanks were only average. And they took steps by increasing production of Mk III and Mk IV tanks to replace the Mk II light tank and upgunning the mk III from a 3.7cm gun to a 5.0cm gun.
Nothing in those two tactical encounters suggests the Germans needed a new weapon type for their infantry. Just better tanks or better AT guns.
My suggestion is that when faced with huge numbers of enemy tanks in 1941, the divisional and regimental assets (a few batteries with 3.7cm and 5.0cm AT guns) of the average infantry division (which did not have 88's) are not sufficient and the panzer units are not able or willing to come to the infantry's assistance. So now they do need something to help infantry cope with tanks. AT guns (either towed or SP) are the obvious solution but cannot be afforded in sufficient numbers due to shortages of steel, rubber and fuel for their trucks (AT guns need to be at least motorized to be effective). So something cheap and portable is needed....