It's a question of low speed, fewer moving parts, and axle loadings. On the SP in SF, when an industry track's switch was in pavement, it was usually a tongue and mate switch. These were OK with the steam engines, but the diesel switchers were so heavy they tended to make the tongues concave. When the wheels hit the tip of a tongue, its heel would jump up about 2 1/2". Bad juju. Seashore Trolley Museum has a streetcar version of one of these tongue and mate double slip switches (shallower and narrower flangeways), which it obtained from the T in the 1950s.