Enigma368

Distance is a factor but not a big one imo. Domestic US and Canadian flights are just much much more expensive than in Europe even for comparable distances. I say this having moved from Europe to Vancouver, Canada. I don't think I've ever seen flights between Vancouver and Seattle (a mere 127 miles) for less than $230 US round trip, no matter how far in advance you book. Usually it is $300+. Same for most other city pairings although I think the Canadian airfare market is even worse than the US one.

In Europe a similar distance between two major cities would be often available for $100-$150 R/T on legacy carriers, and as much as half that on low cast carriers.

Ultimately the biggest difference - as others have said - is competition. Low cost carriers like Frontier and Spirit are not ubiquitous in North America like low cost carriers are in Europe. There is a long way to go before the number of routes flown by LCCs can match the number of routes flown by legacy carriers.

Interestingly Mexico seems to be the most competitive North American market for air travel where 2-3 hour flights e.g. MEX-LAS or MEX-CUN are often available for less than $100 R/T. They have lots of low cost carriers with large route networks.