The discussion that won't die!
As I understand it, and according to the Snopes piece on this that I believe you posted previously, other countries used the WHO test. The US did not, because it's standard US policy to develop their own tests. The US
could have used the WHO test like other countries did--there was nothing but US policy preventing that. Now, President Trump's presidency has been characterised by a scorn for established norms and policies,
when he sees a benefit to himself. This was a chance for him to toss aside established policy to benefit the country as a whole. It didn't happen.
I can hypothesise regarding this, but that's all it would be. Perhaps nobody suggested it to him. Perhaps they did, but he believed that adopting the WHO test would run counter to the 'America First' ideology that has carried him this far. The bald explanation, 'that's not how we normally do things' doesn't really hold a lot of water in view of the fact that 'how we normally do things' has never been much of an impediment to Trump before.