The desktop market is shrinking. Market share != user share. By market, I mean the amount of money that's spent on desktops.
There's the trend to use mobile devices (iOS and Android) over desktops for many functions. It's not necessarily that people are getting rid of their desktops, but they are relying on them less and it's no longer seen as essential to have the latest and greatest on the desktop because the emphasis is now on phones.
Even Microsoft will giving free upgrades to Windows 10 for home users of windows 7 and 8. Formerly desktop OS upgrades were a lucrative source of income. I suspect that it's worth more to them to sacrifice the dwindling income from selling upgrades in order to be able to drop support for the older versions sooner.
Commercial distributions focus on the server as that's what most of their customers are playing them for. Also, server support is somewhat simpler than desktop support as there are fewer varieties of enterprise server hardware than desktop hardware and it changes less often. The typical scenario is that rookie user buys a new notebook and tries to install Linux, eventually giving up in disgust as there's no driver support for a key piece of hardware. The hardware support will probably come in six months, but it's too late by then for the rookie. Old hands know this and carefully research what they buy to ensure that the drivers are available.
There's never been much of a Linux desktop market. Home users rarely pay for Linux support and business users generally choose Linux or BSD for specialized fields or as a cheaper alternative to Windows/Mac for limited function locked down desktops.