Lawrence Dillon criticizes Hammerstein for teaching all future generations that "Sol" is pronounced "sew." Suggested alternate lines: “Sol, the Bottom of My Shoe,” or “Sol, a Slice of Fish Filet.”
Nice snark aside, I can see why the 'l' was dropped. One benefit of movable Do solfège over scale degree numbers is the natural flow of one sound to the next, as (almost) all of the solfège syllables have a simple consonant-vowel combination, unlike numbers. Thus the shift from one syllable to the next is easy, allowing the singing of fast passages. But try to sing "one-three-five-three-one" or "five-six-five-six-five" really fast and the difficulty of shifting from the ending consonants to the following starting consonant becomes evident. While pronouncing "Sol Ti" is easier than "five sev," "So Ti" is even easier. And since most (all?) students don't make the connection between the scale degrees and the starting notes of each line of the thousand-year-old hymn, there isn't the need to keep the exact sound.