Price vs demand...
I suspect the mobile phone market is hitting the same issues as the PC, laptop and tablet markets: the market is saturated, improvements are increasingly iterative rather than revolutionary, competition has sliced away at the profit margins, and there's a sizable second-user market. Worse, while Moore's law reduced the cost of PCs and laptops, mobile phones have actually increased in price as more features have been packed in.
E.g. Using Samsung and Apple as easy examples, the launch price for their base Galaxy/iPhone models were:
2012 - S3: £500. iPhone 5 (16gb): £529
2013 - S4: £579. iPhone 5c: £549
2014 - S5: £600. iPhone 6: £619
2015 - S6: £600. iPhone 6 plus (16gb): £619
2016 - S7: £569. iPhone 6s plus (16gb): £619
2017 - S8: £689. iPhone 7 plus: £719
2018 - S9: £739. iPhone 8 plus: £799
There's a few caveats - e.g. these numbers are from a basic Google search and the price of android handsets tends to drop fairly quickly after launch. Still, it does indicate that the base handset cost significantly increased from 2017 onwards.
Admittedly, most people tend to buy a new handset on a contract, but even there, the monthly cost has risen. E.g the article I pulled the S3 pricing[*] from stated that contracts would be between £34 - £46 a month - and most of these contracts didn't have any upfront charges.
Meanwhile, for the S9, the cheapest monthly contract at carphone warehouse is £48 - and that's with £100 paid upfront!
(though to play devil's advocate: you get a lot more bang for your buck these days, and if you factor in inflation, that £34 in 2012 is roughly equivalent to £40 in 2018. Still cheaper, though!)
Equally, if you're happy with a non-flagship handset, there's much cheaper options that are Good Enough for most people. E.g. the HTC One A9S is just £16 on contract with no upfront cost, and it comes with a 5-inch 720p screen, a 13mp camera, 3gb of ram and a octo-core 1.8ghz processor.
[*] https://www.cnet.com/news/samsung-galaxy-s3-uk-price-and-release-date/