In 1935 UI benefits didn’t cover women or agricultural workers (BIPOC) and the tax was levied on full wages. In 1939, progressives wanted more people eligible for benefits. As a "compromise" conservatives opened up benefits to women and agricultural workers but they wanted a tax break in return. That tax break was implemented by lowering the wage base from full wages to a $3,000 ceiling. Typically, a tax break happens through lowering the tax rate. In this case congress lowered the tax base as to circumvent having to get approval from the Social Security Board.
In 1939, the average salary was $1,338.00. This means that today, the ceiling should be at least $147,264.57 (Massachusetts average salary is $65,680.00).
Pro-business tax cut lawmakers want to sell people on "indexing to inflation" because $3,000 in today dollars is $55,000. As you can see, this is a classic example of how wages have not kept up with inflation, thus creating inequitable tax laws for low and median income people and small businesses. Furthermore, politically speaking, once a metric is indexed, it becomes very difficult to make future improvements. So we need to be very sure that a proposed system would work before locking it in by "indexing".