In Dutch spelling it makes no sense to put a trema (dots) on a vowel right after a consonant. It is put on vowels that come after another vowel, indicating the letters don't form a single sound, but a new sound starts with the letter with the trema.
- ie: one sound
- ië: two sounds, i followed by e
A trema is used to denote the separate pronunciation of two consecutive vowels. The trema is put on the vowel with which the a new syllable begins.