There are several reasons for that and they are all linked to the fact that issuance of this certificate must be verified. The certification authority makes sure that you are the owner of the domain so no-one else can get a certificate for your PKI system.
This verification is currently done with a simple HTTP-based verification when a file is created in a web-server's document folder. This means that if a publicly trusted certificate is not issued, the problem is likely to be:
- Let's Encrypt can't download the required file over port 443.
We are experiencing bigger than expected demand and we reached the limit of certificates, we can issue every week.
If you believe the network connectivity is not a problem, please raise a support ticket and we will help you troubleshoot the problem.