If you're using CentOS 6 at some point the backup script updated to use a newer php and it didn't update the cron job that ran the backup script. Run:
#> cat /etc/cron.daily/cwp
If you see something like:
/usr/local/cwp/php -d max_execution_time=1000000 -q /usr/local/cwpsrv/htdocs/resources/admin/include/cron_backup.php
try running it, it'll probably error out and not execute. If this happens try changing it to:
/usr/local/cwp/php71/bin/php -d max_execution_time=1000000 -q /usr/local/cwpsrv/htdocs/resources/admin/include/cron_backup.php
and run it again if it works, update the cronjob.