The chipsets that enable DP-alt are more expensive, and are simply better suited for laptop low power usage, based on cost.
But... The machines end up costing about the same, and newer desktops have that same capability.
I’m guessing that desktop makers 5 years ago simply had space to add an additional (second or third) Display Port, and it was cheaper to do so. In a laptop, especially as thickness keeps decreasing, space to add a DisplayPort is at a premium, at the same time the small screen drives up demand for additional external display options.
Now that many users have USB-C hubs with HDMI outputs, like me, those users, like me, would like DP alternate mode to be available from their desktop’s USB-C port.
In any event, in MY situation, my primary display runs off the 1st DP port on my desktop, and to run my secondary display, which is HDMI (for best resolution input), I’ll just get a DP-to-HDMI adapter. The USB-C multi-function hub will sit on my desk with the HDMI port doing nothing but grinning at me. :-) Should I wish to send HDMI across the room to a TV, I can yank a graphics card* out of one of my other machines, or, more limited (parallel display of “second DP output only”), get a HDMI switch or splitter.
*Said graphics card doesn’t have 4k output, but said TV is not 4k anyway.