The use of 09L for departures is covered by an agreement called the 'Cranford Agreement'
Due to the close proximity of housing at the eastern end of the runway it is considered that departing traffic from 09L would create an intolerable level of noise for local residents when on Easterly operations.
Therefore, 09L is only used for arrivals unless there are exceptional circumstances; I believe one such instance happened recently following overnight work on 09R that resulted in the runway being unusable for departures for a period.
Although as the link below indicates, there may be some discussion about this.
Recent Dft letter regarding Cranford Agreement