Probably a combination of just providing a service and having good technology to do so and companies which want to sell said technology, I guess?
Everyone enjoys the internet. I might be assuming, but the sort of "if you want services, move to a city" sort of rhetoric that might exist somewhere in the US doesn't really exist for us Finns. We understand wanting to live in the middle of the woods while still having access to basic services.
The Northern part is very sparsely populated, yeah (well not compared to some other places in the middle of huge states in the US but) something like two people per square kilometer, but rural living is pretty common throughout the country, so the whole country understands the need for them, perhaps?
Also, I think a lot of the towers are older towers for just 2g, going back from GSM to NMT, those towers always just being updated with newer technology, again perhaps? (I'm too lazy to research this now.) And the need to have just cellular networks to be able to call emergency services if you're lost deep in the woods has always been a pretty high priority, I think?
The only places you maybe can't get cell reception in Finland are some places in the middle of a few national parks in Lapland.