The taxiways at the threshold of 09L were not designed to have as much holding space or as many entry points as their equivalents for 27L/R and 09R. Hence the sheer volume of space needed to have the taxi-ing traffic spaced out for maximum efficiency to keep runway movements up isn't there, the area is constrained for space.
There is the additional issue that the T5 domestic stands are right at the end of the runway, so any proper development to allow 09L departures on a daily basis will likely be part of the runway three works.
No point in doing the work only to have to do it all again at the end of it.