Spores are better suited to broadcasting because they require less energy and material for the plant to produce. Spores are also very resistant to drying and mechanical damage. The disadvatage is that spores have less resources than a seed, and can only germinate in nearly perfect conditions.
Sporangial plants tend to inhabit biomes with predictable microclimates and few animals to disperse seeds, such as tundra, or mountain-tops.
The predictable microclimate is necessary to assure good conditions, but it need not occur often, as long as it occurs more often than a plant's lifetime.