Suspension of eDNA in water allows for transport (active or through osmotic diffusion) through the system (transport distances are variable and depend on output rates, flow, etc.).
When suspended and transported in water, eDNA can be easily captured, extracted and tested for the focal taxa.
eDNA methods can also be applied to terrestrial habitats, but advantages conferred by aquatic systems are lacking. Hence, design considerations are even more critical and false negatives are more likely.
eDNA has just been applied successfully on Sharp-tailed snake (Contia tenuis). So it can be done but it's yet to be the 'norm'.