Any idea on why these areas of the sidings were decommisoned and built over
This was Brent Yard (not to be confused with Willesden Brent).
Guessing here, but I do know huge amounts of coal used to come into London via the Midland
There's a bit more info in
British Marshalling Yards by Michael Rhodes (1981).
In a section on the London yards, he mentions the "National Freight Plan" of the mid-1960s which attempted to concentrate traffic on fewer yards, leading initially to the closure of Ferme Park, Feltham and Ripple Lane yards.
Neasden Yard on the GCR London Extension was also closed, leading to more traffic being routed via the Midland to Brent Yard:
Heavy coal traffic from Yorkshire and the Midlands was transferred to the Midland Main Line and marshalled at Brent Yard.
...Trains from Toton to Brent were transferred to Acton at the end of the 1970s leaving just a few stabling sidings at the old Midland Railway yards.
Decline of the coal traffic into London along the Midland Main Line must have played a major factor in the decision to close the yard.
Some photos
-A 9F seen with Brent to Toton empties circa 1954:
https://www.flickr.com/photos/davidwf2009/8619309639/
-A view looking north in 1977, when still semaphore signalled:
https://www.flickr.com/photos/loose_grip_99/53426493895/
-1979 view with a 25 on parcels and a 56 which has arrived on an MGR train:
https://www.flickr.com/photos/tunnel_one/24906944292/
-Cracking shot of a peak passing on the main line in 1979:
https://www.flickr.com/photos/54a_south_dock/7775479266/
-Contemporary scene showing Cricklewood aggregate terminal (in the triangle formed by the main line and freight chords to Dudding Hill):
https://www.flickr.com/photos/justinfoulger/53542346237/
On the same (up) side of the line as the now-closed Waste transfer station, there was also a rail-served warehouse for a company called "Jerich" that used to receive paper products in bogie vans from Europe, via the channel tunnel and a daily trip from Wembley yard. It gets mentioned briefly in this 2018
planning document about the "new sidings and train stabling facility" in this area.