For what I remember, most of the DNS root servers used to run Bind9 exclusively. I'm glad to see that this is now more diverse with NSD and Knot also being used (see table 4 in the report).
Nothing against Bind9, but it is almost exclusively maintained by the ISC, so the DNS's future used to depend heavily on the ISC getting the funding needed to continue operating.
Not to mention how much better it is for standards/protocols/standarization to have multiple implementations of the same protocol in real-world usage so we can nail down the protocol in all situations. Bind9 almost ended up being "DNS" itself, which would have been bad overall, and instead we're seeing more diversity which means we'll be able to make better use of the specifications.
That hasn't been true for at least 15 years. I was a k-root DNS operator then, and we ran several software stacks on each cluster in case one had a bug.