[1] https://boingboing.net/2015/08/21/make-your-own-tsa-universa...
Now if you mandate everyone use a government key or escrow their key with the government there's now a single place to attack. Also this place is affected by government whims, staff pay freezes, cutbacks, layoffs. All you need is one disgruntled employee or contractor with access and you now have access to _everything_.
This isn’t some hypothetical scenario either http://time.com/106319/heres-what-chinese-hackers-actually-s...
Or it gets left on a USB key in a taxi
Or it gets left on an un-scrubbed laptop that's put up for government auction.
Or any of the other which-ways that humans get around policy limitations that don't have technical barriers, or socially-engineered to get around the technical barriers.
The more important the key, the more sought after it will be by hackers, especially one which could theoretically decrypt an entire countries communications. It's only a matter of time.