1) Crostini didn't integrate with the ChromeOS accessibility infrastructure, so it wasn't practical for users who required assistive technologies which meant gLinux would have to be supported anyway
2) While there is some degree of support for graphics acceleration for Crostini instances, there's no real way to provide direct access to the hardware. A bunch of people needed to do work that required more direct GPU access (either very resource intensive rendering, or GPU-offloaded ML models and the like), which meant gLinux would have to be supported anyway
3) We were in the process of moving to using hardware-backed machine identity for Beyondcorp, and Crostini had no way of providing that and tying that identity to the host identity (you don't want a situation where a guest VM appears to be trustworthy when it's running on an unpatched host)
4) This was also before the acquisition of Neverware, which meant at the time that a team would need to be built to maintain a build of ChromeOS for generic workstation hardware
This was a few years back, and I left getting on for 18 months ago, so it wouldn't surprise me if this is reappraised at some point.
This has changed/is changing. It heavily depends on your team (especially if you're not in engineering, I don't think most non-engineers would have a gLinux machine or a workstation out of the box). We've developed tools for crostini that would let people access a gLinux environment, I don't know how much I can talk about it so I won't go into details (although it's nothing super secret either), but the bottom line is that more and more engineers these days are able to do 100% of their job on a ChromeOS machine, including dealing with google3 stuff.
I believe gLinux can be installed on trusted machines that you connect over SSH as well as client laptops (where you can't checkout code etc).
That said many engineers/non-engineers at Google use ChromeOS on their client laptops (same with macOS).