Sure, we have Crossrail and the DLR, but the fact is that we haven’t built an actual new underground line since 1979.
Now, you might say that ‘oh, we’re a developed country, we’ve done all that, but the fact is that London’s population is rapidly rising and there are still corridors and areas that are unserved.
Paris built its latest metro line in 1998, and the world has built so many new metro systems since then, so why can’t we in London build a new underground line - not a light rail, not a tram but an actual line.
You may ask about money, but the way they do it in Hong Kong is that they hand over brownfield and undeveloped land to the equivalent of TFL, which builds its own shopping malls and high rise apartments to rent, and gives it a massive amount of income which it can spend on improvements to the network.
Instead of handing over our brownfield ‘opportunity areas’ to private developers to sell to outside investors, we should have, and should be, having TFL develop the lot for prifitable rent to, for example, the squeezed middle who can’t afford to buy but can’t get into council houses either.
And as in Hong Kong, this would encourage TFL to build sleek new lines between these centres.
And guess what? Hong Kong does not have to have its internal projects be subsidized by the rest of China, like the rest of the UK is paying for Crossrail.
The reason why we don't build underground lines anymore in London is because we made it incredibly difficult to do so.
In the Edwardian period, there were a few plans for lines that were never built and their respective corridors are still tubeless to this day. These include the "Morgan Line" or AKA the "Chelsea to Hackney Line" or AKA Crossrail 2, and the North East and City Railway-another Morgan plan, that would have ran from Hammersmith, under Kensington Gore and Fleet Street, then turning north at Bank towards Southgate. There were coutnless other perspective tube railways that were never built, mainly because of shaky private finances, Parliamentary and local council difficult attitudes, wayleaves and problems with other private railway companies.
Between the Edwardian period and the Second World War, the policy was to extend tube lines rather than build new ones. This was because there was more a pressing need to serve London's growing suburbs rather than build new lines themselves. Before WW2, LT had plans to quadruple the Central and Northern Lines, as well as to build a new express tunnel for the Bakerloo Line which would have ran from Baker Streen to Green Park.
Now if WW2 didn't happen, we would probably see larger, more complicated Piccadilly, Northern, Central and Bakerloo Lines by the 1950s or 60s, assuming another world war didn't happen that would give the same post-war affects on the economy and planning like WW2 did. After that, it hard to say what LT would have done in the second half of the 20th century without WW2. The London Ports would still have declined and the Isle of Dogs would still need regenerating, so a tube out east would still be needed at some point.
But seeing as WW2 did happen, LT had plans after the war as you probably know to build a series of tube lines as laid out in the 1949 Working Party plan. The corridors for the Victoria and Jubilee Lines were initally meant to be Crossrail type lines if I am correct in thinking. However, even though the Victoria Line was built, and so was half the Fleet Line, the state of the economy, political will and the change of focus to providing transport to the Doclands would slow down post-war tube building. The Chelsea to Hackney Line was to be constructed after the Fleet Line and would have ran on a Victoria-Milbank-Waterloo-Aldwych-Holborn-Farringdon axis, taking over the Aldwych branch of the Picc. If all went to plan, this line would probably have commenced construction in the late 70s/early 80s, completed by the latet 80s probably and would have been the latest tube line built from scratch.
The trouble was in the late 80s, railway building suddenly took off. The government was focused on building the Channel Tunnel Rail Link, Crossrail and the Waterloo and Greenwich Railway-the precursor to the JLE. They didn't even have enough money to fund Thameslink 2000 in the early 90s, coupled with the economic downturn at the time. LT wanted Crossrail and the Chelsea to Hackney Line built together to offer maximum capacity relief, however the cancellation of Crossrail in 1994 led to the Chelsea to Hackney Line being delayed. It then went through a series of iterations until the present day; in 1995 it was proposed the Chenley would be an "express metro" type line with mainline trains. Then in the early 2000s it was a tube line again (although on some early 2000s Crossrail maps it is lsited as Crossrail 2), then in the early 2010s, it became offically a Crossrail.
So you can say, the reason why we haven't built any tube lines lately is because we've been dithering since the 70s on building the third post-war tube line.