if DALs have a narrow dispersion then why is distance and angle from the sidewalls so critical, as well as the reflectivity of these walls?
Precisely because they have narrow dispersion!
Narrow dispersion speakers, when listening far-field, are typically very finicky about position. Think about an ordinary light bulb versus a flood light - what you see is a combination of direct and reflected light. It is the same with hearing.
To continue the analogy, the lighting in the room will be most even with the omnidirectional light bulb (more reflected light). The light bulb is clearly less sensitive to location/direction. The flood light will be much more sensitive to placement/direction...only with careful positioning can one get the desired reflected light to evenly illuminate the sweetspot. It is the same with speakers - wide dispersion makes placement very easy.