It’s a trial. I believe there was the intention to put WiFi on the trains but that was scrapped, I think in part due to the worry of people just riding around the network to use the WiFi and associated anti-social behaviour, but also the tunnel sections are generally the only place where reception is non-existent (EE excepted) and that’s only for a short period so people should be able to go without signal.
If you’re on EE (or one of their MVNOs) then you will get signal underground, they were the only operator that put in the equipment with the other networks pulling out. There is some legacy equipment which might get you basic texts on the other networks but for the most part to get signal underground you’ll need to be on EE.
It was more down to cost.
Providing usable WiFi on a train is insanely expensive, and for commuter/metro trains it's even more of a challenge as the number of users per car can be in the hundreds.
Ultimately, Nexus made the (correct, IMHO) decision that they're a transport provider and not an ISP, and instead focused their efforts elsewhere.
Providing WiFi at stations is considerably easier and cheaper, so this would make sense to an extent - at least whilst EE have been the only operator to install femtocell hardware in tunnels and stations to extend coverage. Again, this was a commercial decision by Nexus, with them opting not to fund the big 4 cellular networks getting more coverage. I believe the legacy O2-funded hardware that worked for 2G/EDGE coverage has been decommissioned (if it hasn't already, it will be very shortly, as all the networks are keen to kill off older 2nd and 3rd generation transmission sites). I seem to recall it only worked for O2 (and MVNO) customers, with a roaming agreement in place for Orange and later T-Mobile customers too.
If people want cellular coverage from other operators on the underground sections of the network, they need to start complaining to their providers. It's the only way anything will happen without Nexus having to stump up the cash.
