Money, and how useful they are.
The ACOG was simply a UOR (urgent operational requirement) and only a few thousand units were purchased. The SUSAT was on its way out - the radioactive element was reaching the end of its life (meaning it would be useless for low-level light fighting) - but, the Elcan wouldn't be fully avaliable until 2009 at the earliest (

- this being the MoD, we're in 2011, and its only
just started to reach some units, and its about 2/5s more expensive than orrigionally planned...)
And remember, the SUSATs won't all loose their usefulness en-massé, some will remain useful for a good few years, whilst others became paperwieghts back in 2009.
The ACOG was only given to some 'teeth' units in Afghanistan. And even then, those squaddies given them seem to be at liberty to choose whether they use the old SUSATs or the ACOG (the same as the new RiS handguard). The reason is that even the MoD could see it would be completely stupid, and expensive to change all SUSATs to ACOGs - only to change those to Elcans a few 'months' (MoD time) later.
If anyone is interested, Hornbil (on RSOV) have released the lever-mount that the ACOG is mounted on SA80s.
The Elcan is pretty nifty, although I've already heard people decrying the fixed X4 magnification it has like the SUSAT, especially since Elcan also make various optics (including variants of the Spectre DR we're getting)
with variable mag.