Why is random assignment a better way to assign participants to different groups?

Yes, you are right, welcome.

I see, this allows for differences between the two groups to be attributed to the experimental stimuli (i.e., treatment or intervention). Thank you.

The benefit of using random assignment is that it “evens the playing field.” The groups will differ only in the program or treatment they are assigned. Any observed change between the two groups can be attributed to the program or treatment rather than the characteristics of the groups.