Why is it called 'quicksand' if you sink into it slowly?

It is not about the speed of sinking in the sand.  

It is about the sand moving as if it were alive and eating you.

Quicksand can panic someone very fast, however for humans it is escapable. 
I am prepared to survive a quicksand experience through research I have done.  This post was edited by Honey Dew at May 2, 2022 12:26 PM MDT

The sinking may be a slow, arduous process but, I'm sure once submerged, the death is fairly quick... although not so much painless!

They escaped it in your “Blazing Saddles” scene.

I always thought it was because another meaning for quick is alive. So the sand acts like it's alive.

Are you stealing my old questions on purpose?

https://answermug.com/forums/topic/90097/why-do-they-call-it-quot-quicksand-quot-when-it-sucks-you-down

I don’t have an answer but I do remember when I was a kid thinking quicksand was going to be a bigger issue in life than it turned out to be.  

Ah, interesting story: the first British explorer to be warned of it claimed he was "quick and--." Tragically, he never finished his sentence.

I think Jane S has the nearest answer: a reference to the sand being "alive".

The use of quick to mean alive is archaic for normal use although it does appear liturgically.

There is nothing special about the sand itself. The deposit is "quick" thanks to being saturated with water, creating a fluid. I have read the advice for someone caught in it, is to lie flat, and float on it... but I'd rather not put it to the test.

Although grain is dry, there have reportedly been a few awful cases of farm-workers drowning from falling into a silo full of grain.