Improved security. So many sites are trusting Let's Encrypt and have cron jobs set to refresh data from them. If Let's encrypt were comprised or went offline, they are now a huge single-point-of-failure (or worse, single-point-of-exploit?) for all these domains. It's become a kind of monoculture. A more diverse ecosystem of offerings would be resilient to any single attack or failure.
On the flip side, more certificate authorities (who, remember, also have the ability to delegate intermediate authorities) also means more attack surface, because except in the case of key pinning (which is rare to my knowledge) any malicious authority or compromised authority's certs could be used to effectively intercept web TLS traffic. If let's encrypt was the only authority, yes that would be bad in many ways (and i'm not arguing for that) but it would mean less authorities to worry about. Anyone remember the bluecoat scandal? https://www.vice.com/en_us/article/78kkwd/a-controversial-su...
How would adding additional CAs improve security? By the very nature of the CA trust system, each CA is itself a single-point-of-failure/exploit (though certificate pinning and other measures improve this somewhat).
