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Because the script has been exploited before and they do not provide good support.
Your comparing WHMCS support as worse than ModernBill/ClientExec support? While I can't comment on ClientExec, as someone who has used both ModernBill and WHMCS for multiple years back to back I will have to strongly disagree with you.
I feel you may be confusing security with the amount of recognition an exploit had. ModernBill has had plenty of vulnerabilities also and WHMCS far less (only 1?). It stands out more just because it was so recent.