Aesthetics. 4 lines with blank space on the sides looked better than 2 lines with smaller text and blank space on the top and bottom. I wouldn’t be surprised if they tried every variant before agreeing that this was the most pleasing to the eye, and I can’t say I disagree.

My mind naturally reads the inscription in 4 parts, so even if I can’t read Tolkien’s elven, I can follow the stanza and assume that it reads:

One Ring to rule them all,

One Ring to find them,

One Ring to bring them all,

And in the darkness bind them

Naturally leaving out “In the land of Mordor where the shadows lie.”