Because NASA is anchored by USA congress as a job creation program, not a space exploration program. Commercial Lunar Payload Services is not designed to create jobs across all USA and stretch it in infinity as is the case with SLS, so congress does not like it. That's also why they did not give the founding to make it possible to finance two services. Actually they did not give enough money even for one service and SpaceX had to agree to receive payments with delay.

> This decision was based on NASA’s longstanding Option A acquisition strategy of making two Option A contract awards. While it remains the Agency’s desire to preserve a competitive environment at this stage of the HLS Program, at the initial prices and milestone payment phasing proposed by each of the Option A offerors, NASA’s current fiscal year budget did not support even a single Option A award. Working in close coordination with the CO, it was therefore my determination that NASA should, as a first step, open price negotiations with the Option A offeror that is both very highly rated from a technical and management perspective and that also had, by a wide margin, the lowest initially-proposed price—SpaceX.

[0] https://www.nasa.gov/sites/default/files/atoms/files/option-...