What happened to HMD/Nokia between the 2017 - 2019 period?

Back when I was a high-school student in this 2017 - 2019 era, HMD released a lot of different devices, including a diverse smartphone portfolio covering every segment imaginable.

What caused them to drop this strategy in such a short time, and went forward with a strategy focusing on affordable smart-and-feature-phones? For example, the 9 PureView was their last flagship released in 2019, and its successor 9.3 PureView never materialized.

I suppose you can blame it on declining sales - from what I could find, HMD's sales indeed took a nosedive in 2020, but a more concrete answer would be much appreciated here.