I don't have a definite reason you (or anybody in particular) should choose Digicert but I can give you a couple of ideas of where technically they might be a good choice and ISRG (Let's Encrypt) are not.
Firstly there may be policy issues and you can pay Digicert to care whereas you can't pay Let's Encrypt to care about your problems. Meta for example pays (paid?) their issuer to obey their private extra requirements on top of the rules for the Web PKI when it comes to names in the famous facebook.com 2LD.
Secondly trust issues. Obviously for a mainstream browser or similar, ISRG are trusted, but maybe you've got a fleet of Multi-function Printers from 2015 across 54 offices and alas none of them trust Let's Encrypt for the TLS servers. Yes, this was a dumb purchase but you don't have a time machine and the people who maintain this fleet keeps telling you the next version will definitely fix it, so meanwhile you're buying Digicert certificates.
This is admittedly a rare use case, but is needed e.g. for setting up a DNS-over-HTTPS server.
ZeroSSL seems to support IPv4 SANs, but fails to validate IPv6 addresses; I tried emailing their support several times about this but they never replied. I finally got a working certificate via GeoCerts (https://www.geocerts.com/), a DigiCert reseller, but doing so required manual validation. For the record, GeoCerts support was fantastic.
