It's getting built! Large sections of it are nearly finished!
Quite a lot of the cost is the NIMBY appeasement mentioned upthread. Something like a quarter of the line will be in tunnels. Making a slower line wouldn't make that any cheaper.
Connections to HS1/Europe, and to Leeds, Golborne, East Midlands, Manchester and finally even Crewe have all been cancelled so now extra expenditures will focus instead on Euston Station. That's not the large section people were interested in riding. Perhaps Old Oak Common should instead have been tunnelled the same distance through to Waterloo International (whose international platforms are now deleted).
The international platforms are not deleted! They were brought back into use from 2018-2019 to serve the Windsor Lines, which includes the service to Reading - platforms 20-24. That somewhat reduces the congestion at Waterloo; the station throat limits adding more services.
The extension to Euston was supposed to have 11 platforms. Even the reduced scope now being implemented is 6 platforms, I believe. All 11 were required to handle the eastern leg of HS2 [providing bypass capacity for the East Coast Main Line out of King's Cross and the Midland Main Line out of St Pancras], and services to Scotland and Manchester [bypassing the West Coast Main Line from Euston's classic platforms].
Quite a lot of the cost is the NIMBY appeasement mentioned upthread. Something like a quarter of the line will be in tunnels. Making a slower line wouldn't make that any cheaper.