The 19th century was a golden age for orchestral trombone writing, but there comes a time in every trombonist's study when he realizes, after several years of looking, that there is no trombone counterpart to the Tchaikovsky Violin Concerto or a Beethoven piano concerto. No major composer* ever wrote one.
Part of the problem is that it just isn't regarded as a solo instument capable of nuance.
Comment from a NYTimes article:
When he won a competition, one critic wrote that a trombone on the concert podium was like an “accordion in church,” Mr. Lindberg said. A London impresario told Mr. Lindberg’s manager he would go to hear a trombone concerto only when “heavily drunk.”
Today's composers are pretty much midgets compared to those of the romantic era, so don't expect any breathtaking developments in this area, but there is some activity. Four trombone concertos are premiering over this coming season:
The premieres suggest that an instrument that has rarely been invited into the spotlight has finally hit the big time. They also shed light on how solo commissions come about, through a mixture of friendship, personal ambition, dedication to the instrument and, sometimes, sheer ’bone luck.
Link to whole article: In the Back, by the Tuba, a Star Is Born
*Actually, Rimsky-Korsakov, a major composer by any measure (Scheherezade, Russian Easter Overture, Flight of the Bumble Bee), did write a trombone concerto. It should have a sticker on the front that says: "WARNING - Don't get your hopes up. Complete disappointment."