Is it because:

• They are sold at a loss to unfairly compete?
• They are built using slave labor?
• They are rolling spy machines? -All of the above?

That's not only security concern but also labour abuse. BYD's Hungary plant is just another case among many:

Seven-day weeks and ‘debt bondage’: China’s first electric car plant in Europe mired in allegations of worker abuse

The BYD factory being built in Szeged, Hungary, is facing scrutiny after reports of EU labour laws being violated among the Chinese migrant workforce.

As the article says,

Asked what conditions are like inside the site, a colleague [Chinese migrant worker] replies: “Nothing out of the ordinary, when you’re a migrant worker.” His supervisors are very strict and living conditions are “quite harsh”, he says

There are also environmental and health risks, as the article says,

Some people in [the Hungarian city of] Szeged feel as if there are too many unanswered questions about how the factory operates. Many were also concerned about health risks.

“The first thing that comes to my mind is infrastructure changes; as far as to what extent environmental factors will be respected, how will this affect us?” Zita, 55, tells the Guardian on the main street. “As a resident of Szeged, I feel that there was not enough information.”

Not to forget social issues,

Questions remain about pressure on housing and the quality of accommodation for migrant workers. Workers in Szeged told CLW of multiple dormitory buildings on the BYD site, six of which were fully occupied with about 450 people each, with an additional 1,000 staff offsite, bringing the total number of workers to 4,000.

Some staff reported working seven days a week “for full monthly cycles except when heavy rain temporarily halted construction”.

As well as 'debt bondage',

Those recruited through subcontractors also told how they had to pay fees of between £860 and £2,100 for the job. Those hired directly by BYD paid no fees, it said.

“For workers coming from low-income regions in China, these fees may constitute a substantial debt bondage,” says CLW, which has called on Hungary to “strengthen inspections and enforce labour and migration laws” at the plant.

This is a tiny sample.

They don't want you buying them for the same reason they don't want you buying German or Japanese cars. Its a reasonable expectation for any country to try and protect their local economy.

However the Chinese exploit cheap/slave labor, and in turn we've handed them global economic power by allowing our companies (and virtually every other country) to outsource labor to exploit it as well. But that has centralized global manufacturing, which also makes them insanely powerful, and able to manufacture things much more inexpensively.

Add to that government incentives and there's no way anyone else can compete, because they're not doing so on an even playing field.

Who are "they"?