I think the reason that various authorities advised against masks is political - there simply weren't enough and to direct the supply to clinical workers.
Which is damning in that they treated us like children who couldn't handle the truth and were also so unimaginative that they did not consider alternative face coverings.
The government communication in the UK has been lamentable. Confusing, inconsistent and patronising. On the other hand, we had idiots who stripped the supermarket shelves of toilet rolls and canned tomatoes for months despite being implored not to do so. If they had gone after the masks...
Yes we should not have taken masks that would go to the NHS but don't forget those masks should have been in government stockpiles well out of reach. How can we blame and punish the publics self control (or lack of it) when it's a government failure that caused the issue. Had we all bought masks and the state made them instead of redundant hospitals perhaps we would be in a much better place (less NHS casualties and physical/mental suffering).
All in all it just doesn't sit right with me to justify the government's decision without mentioning they were the ones that put us in the position where it had to be made.
There is now, and was then, plenty of evidence to suppose that enough people would not handle the truth responsibly, and would panic-buy masks rather than adopt alternatives.
Is the US government so impotent that it can't ask retailers to ship their supplies of masks to a commandeered warehouse? Of course some would go missing, but substantially all would arrive.
I personally think one aspect that doesn't get much attention is the matter of public education. At the arrival of the pandemic here in Germany the initial advice was to seriously implement hygiene rules: 1.) Wash hands thoroughly and often, 30 sec long, plenty of regular household brand hand soap, 2.) When coughing or sneezing, turn away from others and put your elbow in front of your face and 3.) Keep distance of minimum 1.5 meters and 4.) Limit contact with people outside your home environment wherever possible.
Assuming everyone followed this advice, at that particular moment we didn't need masks so much (most people were in lockdown anyway) and besides the fact that there were not enough medical masks to go around, there was more danger in using generic face coverings than not because the average Joe didn't know how to do so safely and the virus was not yet under any sort of control.
Around Mid-May, we had a different circumstance. The general public had taken the hygiene rules to heart and been in lockdown quite awhile and the sun came out - Spring sprang, the numbers went down (thanks to lots of cooperation from the general public) and that was when the public education effort went in to the next phase: To give official advice and instruction on how to properly handle face coverings in a way that they help instead of hinder.
Meanwhile everyone, even Grandparents, my mother, and some people's recalcitrant Uncles know how to use Skype and Zoom. We wash hands like surgeons, we can and do handle face coverings without making ourselves sick and I think, for a public education effort that started 5 months ago and was directed at 90 million people, that is simply huge. I try to remain wary of the impulse to judge what we did yesterday with what we know today.
That was a colossal mistake. Not only have they ruined their credibility in this pandemic but they've guaranteed that there will be panic buying during all future pandemics. It was a one-time weapon that provided marginal benefit and terrible long term consequences that go far beyond contagious diseases. Don't be surprised if anti-vaxxers use this incident for decades to come as proof that health authorities lie.