The diesels offered in 3/4+ ton trucks are very heavy. The 1/2 ton trucks would not be able to handle those engines. Smaller scaled/lighter diesels could work well in the half tons. Ram offers one for 2014+. Titan 2016+.
Why don't they all have it? I believe Ford, GM and Chrysler were all developing them in the mid 2000s. The economy in the second half of 2000s forced the companies to cut back R&D as pickup/large SUV sales dropped by 60%. The diesel option is $8-9k over the gas engines offered in the 3/4+ tons. It is about $3000 in the Ram 1500 over the Hemi. It can be a market risk.
Ford is going the Ecoboost route and doing quite well. 3.5L for power, 2.7L for balance of power and efficiency.