A possible good reason might be that there is a higher level of fraud (e.g. faked financial statements), or a higher level of public charge in applications from some countries - especially if it is a pause while procedures are changed. On the other hand the true motive might be something else.
That said, I have no idea why its this particular list of countries. Why Thailand or Jamaica or Nepal?
I don't think individualized determination are even possible. Unless you take very few people from each country, they'll inevitably find each other and form communities. And the kinds of communities they form will be driven by their cultures. The question isn't "would this one Bangladeshi be a good immigrant." It is "when 100,000 Bangladeshis inevitably form a cultural enclave in some city, will that be better or worse than what was there before?"
"Immigrant visa processing from these 75 countries will be paused while the State Department reassesses immigration processing procedures to prevent the entry of foreign nationals who would take welfare and public benefits,"
https://www.reuters.com/world/us/us-suspend-visa-processing-...