Why are Let's Encrypt free SSLs so popular? First of all, obviously, because they are free of charge. This is one of the key points among all the benefits provided by the Let's Encrypt Certification Authority.
Second, issuing a certificate when it comes to Linux users is relatively simple. There is an automatic option for issuing a certificate, as well as a manual one. Each user selects the most convenient choice for himself. Averagely, issuing a certificate takes up to 15 minutes.