The effort that it must have taken to lay the "Polar Express" cable for such a thin population -- you think they'd just use satellite access.
Probably a pretty important project to do that one for Russia anyway, it connects all parts of the country.
Way, way cheaper. Trenching or tunneling on the one hand, mostly just steaming forward whilst paying out cable on the other. The cable itself is more expensive for submarine use though, so there would be a point where cost crosses over.
As a standard internet user the difference between 20ms and 24ms might not matter to you but when selling enterprise services, especially around financial institutions (but not exclusively) that 4ms matters. Often it matters enough that the business case for terrestrial cable starts to make sense.