Overhead conductors use air as the insulator. Underground cables use an insulating jacket. In the past it was really difficult to build cables with voltage ranges in the 10s of thousands of volts without additional complexity like a dielectric oil being pumped through the cable. I think modern dielectrics are significantly better though.

Cost, mainly

The cost of oil insulated cables that can do 132kv is about £900 a meter. Whilst there are HV cables that exist on the outskirts of london, they are much rarer in zones 1-3.

I assume that the cost of pylons with raw cables is much much cheaper. The problem is planning permissions, physical clearance. planning permission and now one wants to live near HV cables (that they know of. There are a bunch of 33kv cables buried outside posh people's houses in zone 5, and a bunch in canals.)

Overhead high voltage conductors are not insulated with a coating, probably for many reasons but certainly for cost and heat dissipation.

That means the path through the air to some conducting materials needs a certain distance, and that even when wet or iced over or whatever can happen up there.

Overhead lines need big ceramic stacks because the air is the insulation. In tunnels, the insulation is in the cable itself, and the tunnel just provides structure, cooling, and controlled geometry

The tunnel is too small to use air as an insulator so they use cable assemblies with multiple layers of insulation.

Here’s some high-voltage cable spec sheets that show a cross-section of the assembly for voltages above 69kV: https://www.southwire.com/wire-cable/high-voltage-undergroun...