Why is Thunder considered a bad move?

Because it only has 70% accura... oh wait, wrong game

Right. And Rock Slide is the rock version of Thunder Bolt. Rock Slide is better than stone edge - only problem is that it's not available on anything good currently, unlike TB, which is.

Plus, also unlike stone edge, there's a full bar electric move that does 40% more damage.

Really, stone edge kinda sucks (only two full bar charge moves do less base damage), the only reason we like it is that when you need rock damage, you don't have any better attacks to choose from! With electric, you do!

Having used otherwise comparable mons (lanturn specifically - I used those very heavily vs garys pre gym change) with TB and with Thunder, I can say that the ones with TB were more effective by leaps and bounds.

Because there are better options like Thunderbolt and Discharge.

It is on par with (if not slightly better than) Zap Cannon in raids, though. In normal battles, it is just arguably inferior.

Fireblast was a great move, before the introduction of overheat, fireblast isn't a terribile move, but overheat is much better, if they start giving rockslide to Tyrannitar and golem, nobody Will want stonedge anymore, but since rockslide can be lerned only by crappy Mons like sudowoodo, shuckle and dunsparce, the only viable alternative is rockblast, that sucks, the same goes for earthquake, that is even worse than stonedge, but since there aren't much better alternative is the preferred ground charged Attack (and the only golem and rhydon one) Thunderbolt is better, while wild charge is even better, only the unavaible raikou is the only good Electric type that learns it (raichu sucks)

You are correct. As others have noted, however, it's about context. Electric simply has better options, while Stone Edge is the best of its type on the Pokemon that matter in this environment.