How can Colossus transmute from his armored form of organic steel to human (and vice versa)but Mercury can't???

She's a teenager. She requires angst. Being stuck with a physical attribute that prevents someone from being fully able to engage with society or that denies some aspect of the human experience is one of the most popular ways for writers to give a character angst. See: Cyborg, Captain Atom, Mettle, Hazmat, Rogue, Chamber etc.

Colossus is not a teenager. Angst is not a pressing requirement for his character.

Sexism.

Maybe because she is a teenager and she still doesn't have complete control over her powers while character like Colossus is not a teenager and has had his powers for a longer time.

I believe when her abilities manifested she was completely transformed into liquid metal, whereas Colossus merely gained the ability to transform into organic metal. It is possible that some future writer could have her gain the ability to transform back and forth between human and metal, but that would be detrimental to her character and reduce her to the status of Colossus-derivative. Besides, if she does transform back, she should be missing bits and pieces of herself, due to X-23 cutting of her hands to save her from the Nimrod and the Facility removing several pieces of her in order to create those hunting dog/bear/dinosaur things.

Because she's not Colossus.

Each mutant ability is different. Her power might look similar to Colossus's but its still different.