Why shouldn't they concentrate on the main routes? These are businesses just like any other not non-profits. If the demand was there then they would serve these rural routes but it isn't.
As for comparing them to trains, these companies are not "stealing" riders from anywhere. They are providing a service that clearly the train networks can't.
The point isn't that Megabus ought to serve smaller communities out of the goodness of their hearts, the point is that this inequitable distribution of service is what happens when you depend on private for-profit companies for intercity bus service.
It is also worth noting that the per-passenger cost of train travel drops rapidly with higher ridership. Lengthening an intercity train from 4 cars to 8 doubles the capacity, but the operational cost doesn't increase that significantly. So VIA currently sits in a feedback loop where their prices are high because ridership is low, and ridership is low because prices are high. And they can't temporarily drop prices to capture the market, because they would completely outcompete the private bus companies, who would make a huge fuss about being put out of business by a publicly-funded company. But following an initial investment to cut prices and improve frequency and reliability, VIA's Corridor services wouldn't need any public operating funding anymore, as
VIA has indeed stated in relation to the HFR proposal. This is already the case with some national railway companies in Europe, such as Nederlandse Spoorwegen, which turned
a profit of €215M in 2019. And since NS is owned by the Dutch government, that money goes back into the pockets of taxpayers, unlike profits from fully-private bus companies. These profits don't take into account the full cost of infrastructure construction or maintenance, but neither do the profits of private bus companies.
VIA's operating costs are currently higher than necessary due to their lack of cab cars. They are therefore faced with the choice between wyeing trains at the end of the line, which is expensive, or using two locomotives, which is also expensive. The new Siemens trainsets do include cab cars, which should make the services more profitable.
The bus routes from London mirror the VIA routes for the most part but people won't take VIA because it is painfully slow, highly unreliable, infrequent, and usually more expensive. Saying bus riders are "stealing" riders is like saying the airlines are doing the same who also only provide service to profitable destinations. If VIA offered a superior service then people would take it but they most assuredly don't.
Prior to the pandemic, Greyhound ran 11 buses per day, of which 2 were local trips too slow to be relevant for Toronto-London travel. VIA ran 7 trains per day, of which 2 were too slow to be relevant. VIA's typical travel time was 2h10, and Greyhound's was 2h25. VIA trains were often late, and so were Greyhound buses.
A typical VIA train carries around 4 to 5 times as many passengers as a Greyhound bus. Assuming an average train length of 4 cars, the VIA schedule provides a capacity of 1736 seats, and the Greyhound schedule provides 605 seats. Even if VIA had a lower occupancy rate than Greyhound, it is safe to assume that their market share was more than double Greyhound's.