Probably influenced by the fact ADL and Stagecoach had owners in common.
To me, this speaks to a wider political problem however. Western governments, not just the UK, have sleepwalked in to a situation where China controls a significant amount of the manufacturing supply chain across multiple industries. There has been support from the PRC government for them to invest, and for Chinese to get a good education at Western universities. Rather than recognise the long term threat this presented, most nations have taken the path of greed by taking cheap products, moving their supply chains to China and letting Chinese students support their education system.
No one in power seems to have considered the longer term implications of this, which are many. We've lost countless skilled manufacturing jobs; people who once earned a good wage, paid good taxes in to the exchequer, now stack shelves in Tesco or drive Amazon vans for a living. (Jobs which themselves are under threat from advancing technology) Companies have had little incentive to invest when they can just outsource to China and get parts of equal quality at lower cost. Universities which rebuilt campuses on the back of Chinese student fees are now struggling when numbers have dropped, and Chinese universities have soared up the rankings now they have a local talent pool. School leavers are finding it harder to justify getting in to debt to take a degree when the jobs they'd normally move in to no longer exist within our economy.
While we were talking about cheap Chinese tat, they were building quality and local capability and are now in a position where they can build every bit as good if not better quality products at a lower price. It's not just about ADL not being able to build a bus that doesn't rattle, it's about Yutong having access to battery and motor suppliers who have factories just down the road from them, who source raw materials from local mines, which are refined locally. Even if ADL put together buses that were totally fault free and really well designed, they still wouldn't have the same access to parts and suppliers to be able to match or even get near the pricing.
A tariff on Chinese products would help offset the loss in taxes and the higher social security costs of not having the skilled work here in the UK, but unless we as a country invest properly in rebuilding the end to end supply chains then it's just a sticking plaster on an amputated limb.