SSH is geared towards the Enterprise users. The free version doesn’t support SSH and there are good reasons why SSH is not in the free version.
People expect software to install and work out of the box, while SSH requires that you install and configure SSH beforehand. This is not what the average user expects. Second, SSH can be tricky to configure, so why give users of the free version these problems before they’ve even started? Third, the NX protocol can use both TCP and UDP for multimedia traffic and this can make a huge performance difference.
Clearly NoMachine wants people to get the best from the software. If you are in a setting where SSH is a requirement (but you don’t want to install one of the Enterprise pay-for products), or you’re tech-savvy, then you can still tunnel the NX connection over SSH, as you would do for any other protocol.
If you are interested in knowing more about the differences, I suggest you check this article: https://www.nomachine.com/AR11K00739