RegExp "i" case insensitive VS toLowerCase() (javascript)

I'm hoping someone can explain to me why I need to use "toLowerCase()" if I'm already using a regular expression that is case insensitive "i". The exercise is a pangram that can accept numbers and non-ascii characters, but all letters of the alphabet MUST be present in lower case, upper case, or mixed. I wasn't able to solve this exercise correctly until I added "toLowerCase()". This is one of the javascript exercises from exercism.io. Below is my code:

var Pangram = function (sentence) {
  this.sentence = sentence;
};

Pangram.prototype.isPangram = function (){
  var alphabet = "abcdefghijklmnopqrstuvwxyz", mustHave = /^[a-z]+$/gi, 
  x = this.sentence.toLowerCase(), isItValid = mustHave.test(x);

  for (var i = 0; i < alphabet.length; i++){
    if (x.indexOf(alphabet[i]) === -1 && isItValid === false){
      return false;
    }

  }
  return true;

};

module.exports = Pangram;

It's because you're manually checking for lowercase letters:

if (x.indexOf(alphabet[i]) === -1)

alphabet[i] will be one of your alphabet string, which you have defined as lowercase.

It looks like you don't need the regex at all here, or at least it's not doing what you think it's doing. Since your regex only allows for alpha characters, it will fail if your sentence has any spaces.