Because it is a natural monopoly, essentially due to the massive cost of building and maintaining a second last-mile network which would necessarily be underutilised. The costs of maintaining a last-mile network are essentially independent of the number of users (unless you let unused parts of the network go out of date/fall into disrepair, in which case you effectively don't have a second network anymore).
As such, it is always better to try and win over one more customer to increase revenue, even if that customer pays less than the cost of keeping their line in working order, both because the costs are mostly there anyway, and because it reduces the revenue of the competition, thus increasing the likelyhood that they will fail and thus make you have a monopoly again.
That's actually exactly the behaviour you see over and over when some smaller business (or larger business, for that matter, see Google Fiber) tries to build a network where the monopolistic incumbent so far didn't care to provide useful service. The moment that project is announced, you get really good and cheap connectivity from the incumbent. Given that they before didn't consider it to be a good investment, you can guess that they are doing it only to destroy the competition.
Also, the network operator that has the less-utilized network necessarily has to charge higher prices per user to cover their costs, which leads to a positive feedback of even less customers, leading to even higher prices, ...
Having two parallel last mile networks is effectively unstable. There is a reason why there aren't multiple water pipes and multiple electricity cables and multiple natural gas pipes in your house.
Why do you say it's more difficult? It seems like it would be more sustainable in the long run even if it's more difficult up front.