The 32pdr was not ready in time. The 17pdr was. The 3'' to 17pdr was done in the M10 and lead to a most succeful vehicle. It could probably be done in the M6 with minimal fuss.
True, the 17 pdr in there would not have been hard at all, as some M6 testing had the 90mm in that turret
the 3" M7 gun was based off a WWI design, so it was big and heavy, nearly to that of the later 90mm that replaced it with little modification.
The 3.7"AA gun the 32pdr was based off of, been in production since before the war started, while the 17 pdr was still on paper. No rush to make a Tank gun of the 3.7", since zero platforms in UK (or anybody)could carry it in 1939.
Enter the M6, with its oversized turret that had room for the 3", a coax 37mm, and a 50 cal in the rear, all on a 69" in turret ring, same as the Sherman and later Pershing. In Israeli service, the French 105mm Medium velocity gun was fitted to Sherman turrets.
The M6A2E1 had a 80" inch turret ring to test turrets for Super Heavy tanks, with the largest being a high velocity 155mm gun in 1945
The just missed the war Centurion had 74"
So yes, the M6 was a beast of a big tank, able to carry any cannon the Allies could conceivably come up with.
So the British have an undergunned heavy Tank, and know the Germans have a Tiger. Early in 1942, the 17 pdr just isn't ready yet.
But there are 32 pdr gun tubes, with a lot of AA paraphernalia hung around it, that isn't needed for direct fire
Much faster to change an existing gun, than make new.
And the British were good at shoehorning cannons into existing turrets, like the Churchill NA75
Big Tank with decent armor + Big Gun = happy UK tankers
Sure a 17 pdr would be good, 32 pdr much better