Priority gains more than 2x speed per tier, so this disparity between them is reduced at tier 2 and phased out completely at tier 3. It's mainly for gameplay reasons to compensate for the fact that it's not meant to be running all the time, so otherwise you'd need twice as many of them. It's meant to be a different style of electricity use, not to be much more expensive. Another gameplay reason is that their electricity use is so high that having priority ones on your grid in the early game is actually a burden and this lets you control it at a finer granularity. Particularly since priority is in your starting equipment and at 2x speed it would really tax your initial allotment of solar panels and batteries.
An additional gameplay reason for having this difference at tier 1 is to nudge new players into having a reason to take a moment to try and understand the difference rather than just ignoring the weird one with unconventional behavior. Some players really, really need that. Even with the energy storage checkpoint linking surge to energy storage, and the tooltip description spelling out the difference, some people don't get it. Some people still just push through new techs and checkpoints without paying attention and think there isn't any way to store energy until green science unlocks batteries. Some people figure out you can make fluid batteries, but still try to do it with priority electrolyzers which doesn't really work. Some people figure out exactly how to make hydrogen batteries, but never consider the role that surge could play in their production systems rather than just storage. Having that tangible speed difference might at some point get them to take a second look and figure it out.
In the original beta there were no surge vs. priority electrolyzers. There were only electrolyzers, which behaved like surge electrolyzers. They can be used for anything as long as you understand how they work. But testers really didn't get it or like using them, and wanted "regular" electrolyzers. You can think of surge electrolyzer speed as the intended default and priority electrolyzers as an optional variant. Electrolyzers are meant to be fast at peak operation but not necessarily running full bore all the time.
So this isn't driven primarily by an attempt at realism, but if it makes you feel better, you can imagine a physical reason is that it's harder to maintain a system that's required to be operational 24/7 compared to a system that can build up capacity, sprint for a bit, and then rest, repair, and recharge.
