Hellions could board transports in 7th too, they were just bulky, so the only option was a minimum squad in a Raider.
We had more important things for Raiders to be doing back then, and Hellions weren't as viable as they are now, so almost no one used it.
This world exists because of the things we have done, forever branching to the decisions we make and twisting to what we do not.
”Woe to our enemies. We'll tear them apart regardless.” ~Barrywise