There are only a limited amount of IPv4 IP's available as the limit of this technology has been reached. The follow-up is IPv6, but this version is not supported by every internet user. Support for IPv6 is on it's way, but it will take years until everybody has IPv6 access.
Therefore, it is expensive to have for every domain you own a dediated IP. Instead, you can enable SNI on your server which allows you to have SSL for multiple domains on one IP address. The drawback is that SNI Is not supported by every browser/device, but most modern users have browsers/devices who do support SNI. You can find more information about SNI here.
To enable SNI on your DirectAdmin server, connect via SSH to your server. Add the following to /usr/local/directadmin/conf/directadmin.conf :
Restart DirectAdmin to enable SNI support.
That is it! Also, do not forget to enable SSL for every user in DirectAdmin and install letsencrypt to get free SSL certificates!