Because it is amazeballs?
Joking aside, if you're not into 144hz gaming or need thunderbolt connectivity it is an incredibly good value for money giving what it offers - I paid €580 and it got me...
34" Ultrawide 21:9 Screen
3440x1440p QHD Display
IPS Display - 178 degree viewing angles
99% RGB Coverage, also it comes calibrated out of the box with the profile saved in it (you get a report with it)
5m:1 Contrast ratio
DisplayPort 1.2 Tech (if needed)
USB 3.0 Hub built in
Built in speakers are also decent for what they are
Freesync tech, also overclockable upto 75hz - so for your regular single player FPS games/non competitive stuff it does a great job.
PnP tech
It has some handy software where it effectively splits itself into multiple monitors, amazing for productivity.
Full spec list here:
https://www.lg.com/us/monitors/lg-34UM88C-P-ultrawide-monitor
Speaking from my brief experience on it so far, its a photographers/videographers dream. Amazing colour replication, so much space for tools and viewing space! You can work on panoramic images without having to zoom!
https://i.imgur.com/xx4Socu.jpg
https://i.imgur.com/UHMiU1D.jpg
In short, any other monitor of this size is a considerble hike in price over for small incremental upgrades.
When you start getting to the bigger upgrades like 144hz you're talking a good few hundred more.