Most of the big TLDs have already converted to algo 13 -- .org is still lingering on algo 8, but .com, .net, .edu, .gov have all converted, so a lot of the DNS traffic is using smaller signatures already.

Changing the algorithm for the root is being studied - see for instance https://lists.icann.org/hyperkitty/list/ksk-rollover@icann.o... ; I wouldn't be surprised to see an algo change as part of the next root key rollover.

My guess is they did that to be compatible with FIPS 140-2.

FIPS 140-3 allows ECDSA, but isn't widely deployed yet (among sites required to comply), so using ECDSA would probably cause issues for government organizations that need to use FIPS and DNSSEC.