Here's why:
* The "e" in "pioneer" is part of a closed syllable. A closed syllable is a syllable that ends in a consonant. In "pioneer", the "e" is followed by the consonant "r".
* In closed syllables, the vowel sound is usually short. This means the "e" in "pioneer" has a short "e" sound, similar to the "e" in "bed" or "pen".
So, the correct pronunciation of "pioneer" is PY-uh-neer.