Why site could be defaced?

See the vulnerabilities/insecurities here: https://asafaweb.com/Scan?Url=susanmeyer.info
Status suspicious → HTML code contains hacker’s signature:
Hacked by Hacker Lyly and Murkho Manob from BD GREY HAT HACKERS.
Missed completely here: http://sitecheck.sucuri.net/scanner/?scan=http%3A%2F%2Fsusanmeyer.info

Badness history of IP: https://www.virustotal.com/nl/ip-address/216.51.232.104/information/

Spam check produces the hack: Suspicion of Spam
:56 gmt connection: close hacked by hacker lyly and murkho manob from bd grey hat hackers…
also found: htxp://susanmeyer.info/test404page.js as a sign of being compromised earlier.

Server error:


<configuration>
    <system.web>
        <trace localOnly="false"/>
    </system.web>
</configuration>

polonus