Hoi,

It means that:

• You didn't clean your server deeply, one hidden file might left and wait for activation.
• You may have different extension that may "hole".
• You don't have firewall installed that block popular hacker's request.

This is NOT always only our fault, common. Is not fair.

Ask your hosting provider to scan server (whole) and delete all malware files. Then turn on site again, install scanners and firewall and scan site. You can use ours and others.

Highly recommended: https://htprotect.org/

https://github.com/zkrana/joomla-security-scanner/