I thought it would be easy to swap between Group node and Delete node and use the same expression, but instead I have to use .

Is there a reason behind not having “N” local variable in the Group node?

I found the answer to my own question. It's basically because everything in Houdini is moving toward VEX expressions, so I guess that's why $N is disappearing from some of the modern nodes.

Please correct me if I'm wrong

You've answered your own question correctly but the change isn't an arbitrary decision.

For a great overview of the changes to SOP's and the reasons for the changes check out this Webinar with Jeff Wagner.