Because they were innovative, not despite their unionisation. I don't think the unionisation *caused* the collapse of British industry, it was part of the backlash to its demise. But there was certainly a radicalisation esp in the motor union sector in the mid 1970s that exacerbated the issue, but by then the UK car making industry was already in severe decline, and motorbike industry was already gone.And yet Germany and Northern Italy are still massive industrial powerhouses and America's industrial output is higher than ever and boomed in the 50's and 60's.
Britain's issue RE innovation was also hit hard by the oil crisis - Honda and French/Italian carmakers were already making smaller engine cars that were efficient and used less petrol, in comparison to the British car makers who had barely innovated in years.
Also some good points here about the long term effects of state mandated forced mergers in the sector in the late 1960s that damaged the companies at the heart of the UK sector. Nationalisation did not save the industry either.