why was magma chamber cancelled?

IIRC correctly, it was turned into Dominion because the original Magma Chamber concept spread the game out too far for a conventional 5v5 to be anything but glacial. Which is unfortunate, because the concept art was kickass.

It was also one of the reasons that Riot has generally tried to stop talking about features that aren't yet nearing live deployment, since getting hopes up for something that may need to be redesigned led to bad feelings all around (players feeling like something was taken, Rioters feeling like they had disappointed players, Poros losing all hope and diving into the fountain turret).

Sounds like it wasn't popular enough.

It wasn't popular. I always advocate it to come back whenever and if ever they make URF permanent. Would be cool if URF have its own map and magma chamber would be perfect for it.

The 1v1 map was removed because it wasn't popular. They either released this or something similar and removed it because not many people played it. If I remember statistics they posted they said the first week had a huge influx of players playing it, but a week later it dropped to about 1% of the previous weeks traffic :x.