Why cars such as the Triumph TR6 and MG MGB so slow compared to other cars like the '67 Shelby?

It all comes down to the old adage, "nothing succeeds as cubes, (except rectangular money!)"

The 67 Shelby Mustang had 7L engine that developed 335bhp with 280 lbs/ft of torque. The Kerb weight was 3370 lbs.
This gave a more meaningful 233bhp/ton & 280 lbf/ton
Max power gives you max speed of 135mph, The torque gives you your acceleration which came in at 13.5 sec 1/4 mile & 0-60 of 4.8 secs.

Compare this with the TR6, it only had a 2.5L engine, 150bhp engine (only 125bhp in the US version), 164 lbs/ft of torque. The Kerb weight was only 2491 lbs which gave 135 bhp/ton and 147lbf/ton.
So max speed came in at 120mph and acceleration at 16 sec 1/4 mile & 0-60 of 8 sec.

The MGB Roaster only has 1.8 L giving 95 bhp & 105 lbs/ft Torque in a 2030 lb car
So 105 bhp/ton, 110 lbf/ton gives 107 mph max & a 11 sec 0-60. (can't find a 1/4 mile but I managed 18.5sec out of my friends!)

You can gain an indication from comparing Max Power, Torque & weight (should strictly say mass!) as overall gear ratios & aerodynamics play a significant part in the cars performance.

To answer the last part of your question, both MG & TR's were production line built,

Hope this is of interest & help!