Private chats can contain sensitive material that could be used to extort/blackmail government officials and/or business leaders, and in-app payments can be used for tax evasion, money laundering and bypassing sanctions. That in combination with the fact that TenCent or any large Chinese corporation is essentially an extension of the government means that WeChat can very plausibly become a threat to national security if it gains widespread adoption.

While I agree with your stance that this is infringing on personal freedoms of American citizens, and I too wish our leaders wouldn't always so eagerly infringe on personal freedoms in the name of national security, the national security claim itself does have merit.