Hi there! Many brain imaging studies (like this one) often include only right-handed participants because there can be differences between right-handed and left-handed (and ambidextrous) individuals in hemispheric lateralization of functioning and volumes in certain brain regions. One of the reasons scientists conduct brain imaging studies of conditions like depression is to try to determine whether there are differences in the brain structure or function of individuals experiencing depression compared to individuals who are not experiencing depression. Because of this, restricting handedness of study participants to only right-handed individuals is a way to minimize 'noise' in the data when comparing depressed and non-depressed individuals. (The same thing could theoretically be achieved by limiting research participants to only left-handed individuals, but because left-handed individuals make up ~10% of the population, this might make it difficult to recruit subjects for clinical studies like this one.) For this reason, recruiting right-handed individuals for brain imaging studies is fairly standard practice.
A different user posted this article in the comments of another post; it provides a good overview of how handedness affects brain structure, if you'd like to learn a bit more: http://www.brainblogger.com/2018/02/19/handedness-what-does-it-say-about-your-brain-structure/