Because Water Cooled weapons are harder to displace. A stationary emplacement in WWI was hard to destroy, either a grenade from up close, an infantry support gun being manhandled over no mans land or a somewhat rare tank, so it could stay put. Now tanks are common, so are IFVs with heavy autocannon, most squads have recoilless rifles/RPGs/rocket launchers and Grenade Launchers, guided weapons exist for aircraft and artillery. Ergo a stationary emplacement will have to displace a lot more now, hence lower weight is more of a requirement
Also post WWI you have the development of quick change barrels, which give most of the same advantages without the bulk or complexity, as well as just better metallurgy making overheating less of an issue, which explains why vehicles don't use them, that and the weight and bulk
Gatling guns are mostly used in situations where rate of fire is the main consideration, not sustained fire, to put as many rounds down range in a limited time as possible. Generally you see them in aircraft, small boat, AA, or CIWS applications