The chief obstacle to effective small arms in the earlier 15th c. was the lack of knowledge about manufacturing gunpowder. This is before real chemistry: the only way to arrive at a mostly-pure potassium nitrate, to separate it from salt, and from the calcium and sodium nitrates that often occurred in manure heaps as well, was to simply try things out: to keep track of the sources- what dung heaps produced better saltpeter ( outhouse from taverns that sold wine were better than ones that sold beer) and to add steps to the processing. Eventually, for example, it was found that boiling with wood ashes and quicklime produced more usable saltpeter. Since this had to be figured out with no knowledge of what was actually happening, it took a long time. By the later 15th c., it was- then appeared aimed matchlocks, with long barrels.

With varying rates of reaction in the gunpowder, then, in the early 15th c. there was little point to having aimed small arms- they were simply not going to be accurate. Cannon, small and large, were good against large targets ( like the walls of a castle) or at short range. The Hussites waged a defensive war: they were in rebellion, and Imperial forces were tasked with defeating them. So, the Hussites ( under Jan Zizka) found a way to deploy a mobile defense, chaining together heavy wagons and covering the gaps between them with shields. Smaller-to-medium sized cannon defended those walls much as cannon were being used to defend the walls of actual fortresses. This, combined with crossbows and polearms ( like halberds) made them extremely difficult to take: a Wagenburg formation ( sometimes around 180) was a death-trap for cavalry, and infantry trying to storm the walls of one would not only face the fire from the defenders but also face the cross-fire from others. When the Imperial forces began to deploy their own wagon-fortresses and the Hussites had to go on the offensive, they were often ( though not always) at a much greater disadvantage. And the Imperial forces could keep invading: eventually the Hussites divided into moderate and radical factions, the moderates accepted a peace offer from the Pope that allowed the Bohemian church greater freedom- and ( rather amazingly for the time) that peace deal actually lasted until the Thirty Years War.

TL:DR cannon and firearms were very good at defending walls, so the Hussites waged war with them by making walls that could travel.