Impracticalities
Some types of speakers have such eccentric characteristics that a grille would interact too much with the sound to be practical. Studio monitors, for instance, are required to reproduce audio so accurately that anything in the path of the speaker could obscure aspects of the sound, and thus are rarely seen with grilles.
On the other hand, high powered subwoofers (such as those used in high-end car audio applications), produce such a violent sound wave that a grille may susceptible to rattling or damage while the driver is under load. All but the most sparse grilles also pose the possibility of diminishing or distorting the low frequency waves being produced.
Some speakers simply don't need a grille, perhaps because they are enclosed in a case (such as speakers found inside personal computers) and are not meant to produce high fidelity audio, but only audible tones and noises.
Read more about this topic: Speaker Grille