Interesting, one question i had, was hydroelectric due to mountains/hills glaciation or in the sea like the UK. I googled it, it seems like the former.
Geothermal always seemed to me, it's a pretty difficult source of energy to extract for factories n stuff. Like, a factory can melt steel, but no one wants the ground he stands on to melt as well.
Hydroelectric is a much easier source to work with. Humans have used rivers as a source of energy for thousands of years.
Icelander here, our main power generation is from hydroelectric not geothermal, it's a common misconception.
Also most of the mining here has moved on to easier places like Sweden and Norway. And for cooling / humidity reasons not solely electrical prices.
It was also just foreign owned companies most Icelandic folk don't like it here (I think it makes good use of our remoteness and shit latency however)