Simply because it is more cost-efficient and the project can go live much faster
A solution provider partner is the one who has the ability to sell NetSuite licenses as well as implement NetSuite and provide other professional services for your business. NetSuite partners are the ones who have deep knowledge of specific markets and how things work in those markets.
Although license cost for NetSuite would remain the same across both channels, direct or partner, the implementation is where the difference comes. The Professional services fees of a partner are mostly much lower than that of the Oracle/NetSuite professional services team and the implementation time is much quicker.