Not all efforts succeed first time round.
Dear Mr Van,
Thank you for your interest in exhibiting your work at the YU Art Gallery. I found your work rather interesting based on the slides you included in your original application.
I regret to inform you , however, that at this time we are unable to accept your work for exhibiting at YU. Because of space constraints, the European and Contemporary Art department at the Gallery is unable even to display a majority of our present collection; therefore we feel that we must postpone requests such as yours until we are able to resolve such limitations.
Please find enclose your slides, which you were kind enough to include in your letter.