I've heard GDI engines are kicking out even more particulates than your average diesel.Diesel has issues, particularly particulate emissions. Gasoline engines are getting pretty close in terms of fuel efficiency, and have them beat for particulates and oxides of nitrogen by a lot.
Diesel produces about 10% more co2 per gallon burnt. When diesel was kicking the butt of gasoline engines in mpg, that didn't matter. With variable valve timing and gas direct injection bringing that difference down, it does.
The NOx and particulate issues contribute heavily to smog and are generally carcinogenic.
Add in the near future, diesel doesn't have a lot of options for improvement. Gasoline has variable displacement and camless engines on the horizon.
Then of course there is electric.
Odds are they want to stop incentivizing diesel and start incentivizing plug in series hybrids and EVs.