3 things:
1. Shrinking tech
2. Modular tech
3. "Good enough" performance
The latest Samsung 960 Pro NVMe drive is 3,600 MB/s.  16 gigs of RAM is like $80.  You can buy a 128-gig RAM kit for your desktop if you really want to.  Desktop Intel chips are available with up to 10 cores now.  Heck, I just built a box with parts off Amazon that has two 22-core Xeon chips & half a terabyte of RAM.  It's like cell phones...when your system boots up so quickly & everything launches instantly, what more do you want?  The exceptions being CPU/GPU-intensive activities like DCC, gaming, or crunching, of course.
You can get a tiny little Intel NUC for a few hundred bucks now that you can max out with a 4TB SSD, 32 gigs of RAM, Wi-fi, Bluetooth, Gigabit Ethernet, USB 3.1c ports, etc.  So it's not like you're sacrificing to go small or go to a laptop anymore.  And stuff is going modular now too...I've done some projects with the Razer eGPU dock, which is still pretty dang pricey ($500), but you can plug it into a mini computer like a NUC & have a full-blown GPU:
http://www.razerzone.com/store/razer-core
Which you don't necessarily even need anymore because they shrunk the GPU's too!  Zotac has the EN1060, EN1070, and EN1080 (1080 is water-cooled!), which is smaller-than-a-shoebox cubes that house the new GTX graphics cards.  HP has the Z2 Mini G3 coming out this month in the same form factor, albeit with an optional Xeon CPU, up to 32 gigs of RAM, a 2GB Quadro GPU, and support for up to six (6) monitors.  They sell a turnkey version for $1,599 with a Win7 Pro & Win10 upgrade license with 16 gigs of RAM & a 512gb NVMe drive.  You don't need a giant tower or a huge amount of money to do modeling or pro-level work anymore.
A have a couple different computers-on-a-stick (first-gen Atom & second-gen X5), which are about the size of a Fire Stick or Roku Stick, but run an entire copy of desktop Windows.  Things are getting insanely tiny!  Kaby Lake has shrunk those to the point where the whole computer is now the 
size of a pack of baseball cards & requires a dock for ports because it's so small thanks to 14nm CPU's, SSD's with no moving parts, etc.
In short, the game has changed because technology has progressed.  You can buy a laptop at Best Buy for $249 these days, and while the mechanical HDD is slow, you can swap it out for a cheap SSD & get fantastic performance because even a basic Celeron chip is pretty speedy these days (because it's just a dual-core, non-turbo, non-hyperthreaded Core chip).  Stuff is relatively cheap & small compared to past computers & it's only going to get better & better.  Dell just announced an 8K LCD monitor.  The new OLED television sets from LG are so thin you 
have to mount them on a wall.  Best Buy today, right now, is selling a 55" Sharp LED TV with Roku built-in for $349 shipped.
Desktops will probably never go away because there will always be groups of people who need customizable power (engineers, gamers, etc.), but it's not that big of a deal anymore, financially or performance-wise.  Makes me a little sad because I loved my days of building machines & tinkering with them, but boy is the new stuff sure nice!  I've got a few dozen NUC's sitting on my desk to roll out tomorrow...thanks to my new USB 3.0 mSATA adapter, I can clone over the master image in under 2 minutes.  I don't have to lug a big tower out.  All of the monitors are 28" LED 1080p sets & are super thin & light.  Tech is amazing these days!