It was only during the last 5 years that they finally saw a major decline, mainly due to the rise of Smart Phones in general. Nokia just has never had much of a market share within the US as they did for most of the rest of the world.

While Nokia has pretty much used the Symbian OS exclusively up till 2011, when Nokia partnered up with Microsoft. So in a way they did diversify as they are now selling both Symbian and WP OS phones.

However, the reason Nokia doesn’t just diversify more though is because software control is a major factor in determining margins of profitability.

Mind that companies like Samsung and LG have their own factories, which also produce many other products and so can more easily handle diversity than companies with less diverse/extensive resources.

The Android market is already pretty saturated with “me too” products. So there’s no guarantee that adding Android to their offerings will help them.

While being a “me too” product would arguably make it harder for them to set their own prices or offer anything really unique to set them apart from the competition.

Along with other factors, like any restrictions they may have to work with due to their partnership with MS, the fact that they’d likely have to pay licensing fees to MS for every Android phone they sold, change of staff needed to handle a different OS and having to remake all the specialized apps that they normally include with each of their phones.

Versus now, where MS pays them to make Windows Phones and MS handles most of the software side of things and that helps let them focus on holding onto their declining mobile phone market, which are still big for emerging market regions that aren’t ready for higher end Smart Phones yet.

Meanwhile, Nokia has been investing in developing a replacement for Symbian… Meego, Meltemi, etc. It’s just none have yet gotten very far before being canceled and Nokia moved on to the next seemingly promising new OS they could develop.