Because "depth Interviews" may be seen as a little intimidating and put one on the spot and placing pressure on them, therefore one may feel uncomfortable, and also with depth interviews the participant may lie about a certain question due to them feeling uncomfortable or desirability bias, which just means they lie to make themselves look good. Whereas in a "focus group" you're not alone and therefore others will be with you so you may feel more at ease, and the participant may feel it is easier to open up as they are not pressured as everyone is doing somewhat the same. And plus if one person gives an answer that another participant may have felt uncomfortable saying then it shall decrease social desirability biased answers because if one participant tells the truth, then the others will feel as if it is okay to do so.