The reason consists entirely of "it might pose a threat to national security in a manner that has not yet been realized and we're not actually entirely sure what it would be but it definitely could so nyeh," which is incredibly vague and could be used to justify banning pretty much any foreign electronic device or software. It's really never been anything more than Trump trying to score anti-China points with his base by "hitting back" at them in completely meaningless ways. He's just poking at a low-hanging fruit.
Or Trump claiming Kaspersky spied on you just because its Russian. ---