The root word of 'commercial' is 'commerce' which ends in a vowel, so the 'cial' spelling is still used.
The 'tial' spelling is usually used after a consonant. For example, with 'potential', 'tial' has been added after the consonant 'n'.
The 'cial' spelling is usually used after a vowel. For example, with 'special', 'cial' has been added after the vowel 'e'.
Beta