Maybe it's just to be different.

so he can move

That's the way his character was designed. It's as simple as that, really.    

I believe I've seen Ultimate Colossus with lines before

& if it wasn't he would look somthing like this: 

not segmented, creased. hence why his face isnt creased as theres not much movement there.
 
the funky thing to me was always his hair. his fingernails dont appear when he's changed.
also it remains black, and doesnt change over with the rest of his body (im thinking to give him a Frankenstein-esque look?). a friend and i had an interesting conversation about him actually growing out his hair and changing his style, to the point of weaponizing it in armored form.
imagine big heavy metal dreads...with spikes all over them

Think it was just to be different and a smooth colossus does look better to me then a segmented one. Over all though it has nothing to do with his mobility or anything else its just how he is drawn.

" not segmented, creased. hence why his face isnt creased as theres not much movement there.  the funky thing to me was always his hair. his fingernails dont appear when he's changed. also it remains black, and doesnt change over with the rest of his body (im thinking to give him a Frankenstein-esque look?). a friend and i had an interesting conversation about him actually growing out his hair and changing his style, to the point of weaponizing it in armored form. imagine big heavy metal dreads...with spikes all over them "

"not segmented, creased. hence why his face isnt creased as theres not much movement there.  the funky thing to me was always his hair. his fingernails dont appear when he's changed. also it remains black, and doesnt change over with the rest of his body (im thinking to give him a Frankenstein-esque look?). a friend and i had an interesting conversation about him actually growing out his hair and changing his style, to the point of weaponizing it in armored form. imagine big heavy metal dreads...with spikes all over them "

In 616, his armor is actually segmented, his face isn't though. In Ultimates, his armor is smooth.

When they first made him they probably made him that way to show how he could move around and in 1610 they just decided to go without.

" That's the way his character was designed. It's as simple as that, really.     "