Why was it decided to use a new record type of CAA instead of using TXT?
Using TXT to convey policies has been something that have been in use for a long time, for example “v=spf1 +mx -all”.
Same with SMTP-STS.
The main problem of introducing new record types is that DNS providers & web hosts need to implement support for these, both in their DNS servers and/or their web interfaces.
Using TXT for this, example: “v=caa1; f=128; t=issue; d=letsencrypt.org;”, gives the same advantages, but also the advantage that any DNS operator supporting TXT, will also support the new policy.
the record type 99 (SPF) was depreciated for this very reason.