Here's why:

* Cosmic Abundance: Hydrogen is the lightest and simplest element, making up about 75% of the mass of the observable universe.

* Formation: Hydrogen was the first element to form after the Big Bang.

* Interstellar Gas: The ISM is primarily composed of gas and dust, with hydrogen making up the vast majority of the gas.

* Other Elements: While hydrogen is dominant, helium (He) is the second most abundant element in the ISM.

Therefore, even though the ISM contains a wide range of elements, hydrogen holds the title of the most abundant.