US and Foreign endowments of factors of production were different.

US and Foreign technologies were different.

The US was capital-intensive at the time.

The data that Leontieff used was unusual.

the planned obsolescence of some product categories

a stylized fact of the Heckscher-Ohlin model

how new products are first produced in developed countries and later elsewhere 

the time required to negotiate international treaties

A quota does not harm domestic consumers.

A tariff raises the price of imports more than a quota.

A quota reduces the quantity of imports more than a tariff.

A tariff does not harm foreign producers.

A tariff generates government revenue, while a quota, unless it is sold, does not.