The Suikoden World's technology has stagnated because most countries depend far too much on Runes. Harmonia guards it's secret advancements under lock and key and the dwarves from the North Continent (Falena's dwarves don't seem as advanced) never leave their realm and never share their technology either. Only way for countries to redevelop technology is for Sindar scholars to make more substancial progress and recover more of their lost technology. But so far, it seems Harmonia ad the dwarves keep a monopoly on Sindar secret technology.
As for Rune Cannons, remember Rune cannons do not only fire shells from another dimension. The Rune Cannon themselves are from another dimension. I once posed the question of why Lino en Kuldes kept the Rune Cannons if he knew they had alternative uses. So far Only Kooluk once stumbled upon this new use for the Rune Cannons, but you would think someone would eventually stumble upon it once more. It will be funny when we see the Island Nations, now realizing no more Shells will be found, using their Fishmen dispensers to turn entire armies into smelly fishmen.
The Dwarves do trade their technology with others, the Scarlet Moon Empire purchased their superior armour and the Toran Liberation Army purchased the Fire Spears from them.
Also read here from Suikosource:
Chief of Dwarf Village
Origin: Dwarf Village
Events: Gate Rune Wars
Unlike his elven counterpart the Chief of the Dwarves is not intolerant to other races but he does think that dwarves are superior. He could not believe that any human could have stolen anything from the Dwarven Vault, let alone the plans for the Burning Mirror, so he set a test to see if the hero and his party could repeat the feat. When they returned with the object he had set, the Running Water Root, he pledged his aid to destroy the Burning Mirror. This he did so with the aid of the Firewind Cannon, but not before the mirror had been used to raze the Village of the Elves. He stayed in the sidelines throughout much of the rest of the war but was there when needed. Following the end of the conflict he works with the new Toran authorities; Kirkis, the new chief of the elves; and Kuromimi, the new chief of the kobolds, to bring about greater understanding and co-operation between the different races of Great Forest.