Why rocket weapons developed in fits and starts

Why did rocket weapons have a more stop-start development history than guns, mortars, and other support weapons?

Rockets combined high cost and field-ruggedness concerns with effects that were most useful in major wars, while guns and mortars offered more routine control outside those conditions.

Rocket development depended on cost, durability, and mission demand. Rocket systems had to become cheaper and rugged enough for field service, while their most valuable effects often appeared in major-war conditions instead of everyday operations. Mortars and conventional artillery remained attractive when commanders wanted weapons that were easier to control and employ across a wider range of situations.