There is no easy answer to that question.It is highly doubtful that a single issue led to the demise of the Nissan Titan XD 

At this point, sales have been lackluster, marketing strategies have failed, and consumer expectations have been met. Unable to respond

 we can only speculate that perhaps even the business partnership between Nissan and Cummins didn't work out.

When Jalopnik raised the question of a partnership, both Nissan and Cummins responded mildly 

but failed to communicate their hopes or needs for future collaboration.

did not fare well across the US market. According to Good Car Bad Car, Nissan sold just 21,880 Titans in 2016 

 Nissan sold a total of 52,924 Titans in 2017 and 50,459 in 2018. 

These numbers include Titan and Titan XD. 

Broadly speaking, in 2021 Ram sold 569,388 of his pickups 

This represents a 1% increase from his 2020, when Ram sold 563,676 of his pickups. 

 Meanwhile, US sales of the Ford F-Series hit 140,701

units in Q1 2022, down 31% from Q1 2021, when Ford sold his 203,797 units. 

Overall, the failure to differentiate the Titan XD from

the competition was probably the biggest problem as it impacted sales and profit margins.