They don't understand that choral singers need to sing in a different way.
I was at a choral workshop John Rutter conducted (!!!!!!!!). He was able to communicate this by playing a boychoir tape. Then he explained that the pure, clear, non-vibrato tone is one (extreme) end of a continuum, and that many choral pieces are arranged with that tone in mind... that the effect when that tone is approximated is of bells ringing. Maybe a good recording illustrating this would help. Other tone colors in that continuum give other effects. In no case is the scoopy-swoopy thing wanted-- but asking what IS wanted might help eliminate it from one of the possible approaches. Anyway, a good choral director will specify what tone and color are desired, and will coach on technique to achieve this.
In one large community choir I was in, with a gamut of skills from Chicago Symphony Orchestra Chorus members to little old vibrato ladies, and all ages and levels in between, one of the CSO Chorus members had a neat trick for raising everyone's level of skill. He would ask, during rehearsal, what the conductor WANTED. "What tone do you want from us for this piece? Clear or rich?" etc. The replies were often very helpful in bringing that blend into being. The more skilled singers would sometimes be asked to sing a line or two while others listened, to hear what (and who) they should blend TO.