Presumably they convinced them by paying them money. :D And if they didn't do it, someone else would have.
It seems IdenTrust has not been killed off.
1 IdenTrust 48.5% 53.6%
2 DigiCert Group 13.1% 14.5%
3 Sectigo (Comodo Cybersecurity) 12.1% 13.4%
4 GlobalSign 6.1% 6.7%
5 Let's Encrypt 5.8% 6.4%
6 GoDaddy Group 4.8% 5.3%