Why? What makes one better than another?

I guess this is mostly a Glock question because I can't think of any other companies that make polymer pistol mags.

Why did Gaston feel this was the way to go, and why has nobody else gone in that direction?

This is a serious question, please keep the GD'ness to a dull roar for this one. Thank you.

Glock mags have a steel insert.  They're polymer overmolded.

I know. Still curious.

And didn't Glock go with a 100% polymer design initially?

My question is this: If you look at Magpul, you can see that polymer is indeed a viable option, at least for rifle mags.
At first, everybody balked against the idea of a plastic mag in their rifle, but naysayers be damned, the things work great! And now, lots of companies make a polymer AR mag.

The Glock mags are also famously reliable.

What gave Gaston the idea to try polymer in the first place, and why haven't any other manufacturers followed suit?

The original Glock polymer magazines swelled badly and wouldn't eject reliably umless they were at or near empty. Bt the time Glock was done redesigning their magazines they ended up with a steel insert and feed lips. I'm guessing everyone else looked at the issues, Glock was having and decided to use steel.

Magpul uses a much harder polymer than Glock does. If you drop a polymer AR magazine on a hard surface you can crack the baseplate and thd the mounting tabs. Glock magazines are much softer and don't usually have this problem.

That's a fascinating side note. I've never heard of that happening before. Are you referring to pmags or some other brand?

I thought that was a design goal.  Fully / mostly loaded mags dont drop, empty / nearly empty mags do

It used to be a fairly common euro design feature for an empty magazine to not drop free. The early Glock magazines were designed to do just this. What I'm talking about is when the magazine swells while loaded and can't be removed easily.  I had some really old G23 magazines that had this issue.  Load more than six rounds and it was a real pain to get it out of the gun.

Pretty sure it's economics, plastic is generally cheaper and easier to make complex forms from.
I would think molds would last much longer than dies. And it should require less and easier machining and no welding. The insert is going to offset that a bit, but probably not much.
Of course Gaston had a long history with polymers before he ever made a firearm, and doubtlessly understood the efficiency they can bring to manufacturing, couple that with accounts he's quite obstinate, and you end up with a polymer mag with a metal insert instead of ditching the polymer when it sucked and having someone stamp them out of metal.

nevermind.