Because they had to use up so much more money re-doing the bad art in the movie, naturally they want to hype it up with a campaign, hoping to get people to see it in theaters to make the money they have lost back.
On Twitter, it launched a 12 Days of Sonic give away type thing, along with banners like this. This one is Day 6, and features tees.

The first tee is white with a stock art style rectangle portrait, and the movie's logo beneath. Notice how the art is trying to portray* him "as if he were Modern Sonic" but then it's just a lot of jaggy lines to represent how hairy he is.
The other tee is all black, and has a CG portrait of Movie Sonic as it's only decoration.
*Portray- The non-cg art is clearly being jimmied to take away/minimize the look of the 'movie aspects' like the 2 eyes, odd cat ears, and kind of gummy lower face area to make him more clean-line and 'modern Sonic' looking because that is what's more appealing.