One of the problems was that the top management of Leyland was used to building trucks. They didn't know the car industry. The management of the combined car sections were all Triumph men.
Notice that within a couple of years of Leyland closing Zetland most of the cars they were trying to push in Australia were Triumphs. (Dolomite, 2500, Stag, TR7.)
They had a lot of companies building similar cars for similar markets.
Look at the MGB, MGC , Austin Healey 3000 and the TR6.
Common sense says that they had too many cars similar competing for the same market (the USA market) So what did they do? Kill the Austin Healey (and saved paying Donald Healey royalties similar to what they paid John Cooper) Killed the MGC, killed the MGB-GT-V8 (just when it was looking promising) and eventually killed the MGB.
What did they keep? Triumphs.
Sure they had to cut costs but the cuts always seemed to be in the direction of former BMC products.
They claimed that the Cooper name caused increased insurance costs but the GT name on anything also raised the costs.
This was why the Hillman Hunter GT had its name changed to the Hillman Hustler.