The 50mm PAK was the replacment for the 37mm that the Germans took to sarcastically calling "the Army's door knocker". Yes, the Matildas put the SS Brigade Das Reich into a tizzy at Arras in 1940. Even Rommel got excited and reported that he was being attacked by armoured divisions, not a battalion or two of the Royal Tank Regiment. It's been said that Hitler and the OKH were more anxious about an Allied counter-attack after Arras, and so were more cautious than they needed to be.
The British had the 3.7" AA gun, a bit more powerful than the 88 even and it was tried against tanks and bunkers in the desert with deadly effect, but rigid minds insisted it was an AA gun and shouldn't be used for other purposes! The officer who was pushing the idea happened to be transferred out of the theatre and "business as usual" continued until the 57mm 6 Pounder arrived. Inter-branch politics and inertia basically. Vickers built a 75mm multipurpose gun in the 30s that would have been ideal, but of course rigid little minds don't like guns that don't fit into neat little organizational charts, and so they didn't buy any.
Purple has summed up the other problem: lack of proper inter-arms integration. Again, orthodoxy, apathy and inertia; they never even reached the level of integration that was achieved in 1918, at least not until 1944/45. No wonder Churchill used to rage against the Army leadership in cabinet meetings!