The answer to the question is really quite simple, BL truck division failed because people stopped buying the products they produced. The reason they stopped buying them was because they were not as good as the competition.
Robert has mentioned the positive reports of BL trucks against the foreign competition, but these were ■■■■■■■ or RR powered trucks and the home grown assemblers could do a better job of wrapping up a ■■■■■■■ or RR engine than BL could, so even though BL finally had a truck capable of putting up a good fight, it was outclassed by ERF, Foden and SA, especially when it came to build quality.
However the most important factor was the fleets that ran BL trucks, think back to the big Leyland fleets, they were by and large nationalised companies, BRS being the main one and even they gave Leyland a wide berth, preferring the Guy Big J and latterly their ‘own’ Crusader, when the BRS became the NFC/Excel etc there was no requirement to buy British and they didn’t anymore for the most part. Of course there were other fleets that ran Leyland trucks, especially those engaged in work for BL, the oil companies mostly ran Leyland too, until ERF, Foden and SA got a foot in the door and then they pretty much all stopped buying Leyland until the T45 hit the roads.
In the van and light truck market the antiquated BL products were no match for competition, the Transit was unbeatable and in the 7.5t market the LN2 Mercedes Benz set a new standard and the Roadrunner didn’t do as well as it should have because of that, although the Roadrunner was a very good little lorry and one of the few success stories of BL.
In the middleweight division the lighter end of the market was dominated by Ford with the Cargo and after MB had revolutionised the heavier end with the 1617, the Volvo F6 and Scania 82 soon followed and there wasn’t a BL product that could compete with any of them.
Multi axle rigids were a strong point and the T45 range had a good product in every sub section, from lightweight 6 wheelers for mixers and aggregates up to heavy duty eight wheelers thanks to the Albion and Scammell DNA in the respective products, but even so they were hemorrhaging sales to the foreigners who had final got their heads around the UK multi axle rigid market and had products every bit as good, if not better than the BL offerings.
And that was that, not many people bought BL trucks through choice and those that were required to buy British had a little more choice and few chose BL trucks and who could blame them, after years of inadequate investment the trucks were not that good, the memory of the flawed engineering in the 500 series and the V8, terrible build quality and long lead times from an indifferent work and sales force had all taken its toll.