Snort checks network traffic whereas crowdsec checks logs.
For example some malicious attack patterns in the traffic would be recognized by snort and not by crowdsec as it is logged as common HTTP access.
On the other hand brute-force attacks without success will be recognized via logs by crowdsec. Snort only gets that there was some HTTP login access.
See also Suricata vs CrowdSec | CrowdSec