А в чём причина?

This is an extremely long and complicated story, where one has to understand both the local problems and the national authorities.
The plans were made and implemented simultaneously by the Belgian authorities for Antwerp, Brussels and Charleroi. When the plans were made, there were 2 transport companies operating in Charleroi, which divided the city as it once was in Berlin: one side each. In 1974, one tram company was dissolved and the transport company continued to exist, albeit only with buses.
When the metro line was completed in 1985, it turned out to be in the sector where trams no longer ran.
This led to huge disputes, so that the inauguration had to be cancelled a few days in advance: the transport company with trams was not allowed to enter the sector where the line had been built...
In 1990, both companies merged... but then the local authority also became responsible for public transport.
They immediately saw that they would never be able to bear the operational costs: the calculation was quickly made.
2021: Local authorities hope to get European money to renovate the whole... A new hospital could provide opportunities... Whether this line will ever be in service, the future will tell. In the meantime, a tram runs on this line about every week...

Судя по целой контактной сети, она всё это время была законсервирована?

The most necessary maintenance was always done and also repairs made when necessary. One track was always kept in running order.
Stealing the overhead wire is not a good idea: it has always been under power.
What was not under power (security, lighting stations, etc.) was stolen and has never been replaced.
If a tram is running on this route, it will be under the responsibility of the dispatching, which will of course ensure that no other tram would be sent on the route at the same time.