It is quite expensive, the heating reserves are finite and when you look closely at the details there are a lot of challenges. It isn't quite as easy as putting a pipe in the planet and pumping heat up (even though that is how it is portrayed).

> Why didn't it become the default way to generate power?

It's because unlike nuclear technology, drilling technology has advanced massively. It was spurred on by natural gas fracking, trying to get very hard to recover fossil fuels as the easy stuff ran out.

So there's an entire industry around this new technology that already exists but which has not yet been applied to geothermal. And for a long time there was little no no growth in electricity demand, which greatly suppresses the demand for new generation technology; basically everybody was competing to replace the natural gas, coal, and nuclear generation plants that were reaching their end of life.

Now, the hype of AI has made for a great excuse to build a bunch of new technology, which brings a flood of new investment money, political will at Pubic Utility Commissions, and end-users looking to sign PPAs to secure electricity which helps raise that money for new builds.

If you go based purely on when the technology was developed, sure, deep drilling is more difficult than nuclear fission, and in turn uses tons of new technologies at its base.

And until this new drilling technology was developed, fission was cheaper, but it won't be cheaper for long. Drilling is a technology with a learning curve which means that costs fall as we do more of it. Nuclear has not had a significant learning curve, and in fact in many cases it has gotten more expensive as construction labor gets more expensive over time.