Today Samsung has decided to bring the 128GB and 256GB Galaxy S9 and S9+ to the USA. Both iterations will become available to pre-order tomorrow, on May 1. They will be released on May 18, exclusively at Samsung.com.
We assume the company has surveyed the market and figured out there wouldn't be a lot of demand for these storage versions. Otherwise, selling them exclusively through its online store wouldn't make a lot of sense.