I want to say it's because the output jacks are installed on the narrower sides of the guitar as opposed to being placed on the pickguard or "front" side of the guitar. However, I've noticed this isn't the case with a lot of Sidejack guitars like the Sidejack Pro JM, Sidejack 300, etc. I'm pretty curious about the namesake of these guitars myself, actually. (:
@Dudër yep, you're on the right track about the location of the jack. Earlier Mosrites featured the output jack on the side of their body, and when vintage prices started to rise, these ones were worth a LOT more than their newer counterparts. They eventually earned the nickname "Sidejack" to differentiate them from later releases, which featured output jacks mounted to the top of their pickguard. As such, the name "Sidejack" was given to the first of our Mosrite tributes as a nod to this nickname.
Over time, what was once just a model name (Sidejack) under the Eastwood umbrella has grown into a full-on series of guitars, housing all of our Semie Mosely-inspired builds. So, as far as Eastwood Sidejacks go, the name no longer has any ties to the jack's location. It's just a name that we like, and harkens back to the pinnacle of the original guitar in history.