(Reopened the original topic and merged the previous posts.)
Compared to OCSP, CRL is quite the bandwidth hog. It’s essentially a file containing a list of all revoked certificates from a particular CA (though some partitioning is possible to improve performance), and every user needs to fetch the whole file. Cloudflare had a blog post on this a few years ago during the Heartbleed fiasco (during which a lot of certificates were revoked). They estimated the bandwidth cost for the revocations of just one CA at about $400,000.
Let’s Encrypt probably has (at least) an order of magnitude more active certificates, so we might be talking about $4M for the next Heartbleed. That’s more than what it costs to run Let’s Encrypt for a year otherwise (about $3M). Even without a similar event, the cost for CRL would be significant - about 25,000 certificates are revoked each month (based on stats from December).
Technical reasons aside, it’s probably simply too costly.