Wide bore stuff is mostly foreign, that's why. It's also hella expensive to do right (because you will need other upgrades and time to make it work properly).
Basically it originated in Japan (like most Airsoft stuff) where their site limits are very rigid (indeed,
lawbinding) and rather low (regardless of auto, semi or bolt action. People started upgrading to wide bore barrels and offsetting the loss in FPS to bring their guns back up to legal limits (and benefiting hugely from significantly increased range). In the UK we don't really have that problem to such an extent - most people just want their FPS to be a bit higher for a bit less drop-off. There are so many tight bore manufacturers out there these days that the barrels are very cheap and generally of very good quality so most people go for them.
But yeh, tighter is not always better and anyone that tells you that has probably never bought anything buy cheapo TBBs. It is true that for many tight bore is better, but if you are going to put the effort into a new nozzle, cylinder, spring, gears, battery, motor, hop and barrel then wide bore
will win on range and accuracy. Granted that you're also happy to R-Hop it (or use some other flat hop).
And yep, you're completely right - tight bore is about gas (be that air or propane or anything else) efficiency and spitting that BB out at the highest speed possible, but BBs so rattle in the barrel no matter how close to 6.00 you get and that's why flyers and other things happen. I can't comment on how much speed you'd lose, but it will be significant.
Just don't upgrade only your spring or you'll have a very bad day indeed when something else chooses to fail catastrophically during a skirmish. You will also need a new nozzle/cylinder for higher output anyway and a new motor to drive that spring. Probably the only thing you could get away with not doing is changing the gears, but make sure you're well shimmed. Basically, be ready to commit a lot of time and money to a wide bore barrel or don't bother - It's a lot of hassle. Many people haven't the expertise, time or cash to deal with this and so they just go down the tight bore route (which does give a clear improvement in performance for very little work or money).
Many will also claim that tight bores 'give better accuracy': This is very true over a stock barrel in your average AEG (which tends to be made from pot metal) but almost every aftermarket barrel (no matter how cheap) is going to be smoother and made to tighter tolerances than the one in something stock - thus resulting in improved accuracy.
