The GEML has a unique set of circumstances which means the track gets hammered more than anywhere else on the network:
1) frequent high speed passenger services (100mph) - which needs a good alignment
2) frequent heavy freight services on the same tracks (an average of 2 freights an hour each way, usually at 75mph, which batter the tracks)
3) largely built on London Clay which is a cross between jelly and a sponge.
Therefore the track needs replacing often, and maintaining very regularly, to keep it in a reasonable condition.
Having done my fair share of track renewals between London and Norwich, what comes out is usually a messy gloop mix of ballast, liquified clay and, ahem, all the things that get flushed out of train toilets for the last 30-40 years.
As others have said, on top of this is all the work required to replace the late 40s overhead line kit (now done) and lots of work for Crossrail. Coming up - replacement of all the point work at Witham, and building the new station near Chelmsford.