Because long wavelengths require physically large sources in the plane of interest. I just love it when any manufacturer quotes horizontal pattern control in boxes with a single 10 or 12 " LF driver. Doesn't anyone read the coverage plots anymore?
Exactly.
It's simply the case that for a single cone driver on the front of a box, with no horn or wave guide or anything in front of it, there is simply pattern control as such. There is nothing there to control the pattern. The driver will have a very wide dispersion at lower frequencies, and this will progressively narrow until the wavelengths start to become of similar size to, and then smaller than, the size of the driver/cab. This is just the natural dispersion of any reflex loaded driver and is just a function of wavelength vs size.
With the usual single driver, speaker on a stick tops, or with single cone driver line array cabs, you can only hope that when the speaker was designed they took dispersion into consideration and chose a crossover point and processing that worked to control this as well as just based on frequency response. The actual degrees figure quoted will usually refer more to the HF horn than the cone driver.
With bigger line array cabs this obviously becomes a little easier at the low end. The usual dipole setup of a driver on each end of the cab controls dispersion to a much lower frequency. But then you start to have problems at the upper end of the con driver's range, and their dispersion also narrows and begins to beam and lobe much earlier. Often as someone said this is overcame by rolling off one of the drivers early, or by changing over to smaller drivers at a suitable frequency.
But it still applies that pattern control is a pretty nebulous thing, dipole arrangements can only do so much. Even in large line array cabs, the dispersion quoted will often refer more to the HF horn or wave guide than the cone drivers, and the dispersion will vary throughout the range of the cab.
If you really want/need pattern control over the full range of the box, then you needed point source boxes. And big, fully horn loaded ones at that if you want that pattern control to extend down to as low a frequency as possible. While line arrays are sexy in the eyes of bands and promoters and that's what they will often spec, as people have said it doesn't seem like the right tool in this case
Some of the Danley boxes would be a perfect fit for this venue in that respect, and big hornloaded boxes like the EM acoustics X3 (
http://www.emacoustics.co.uk/docs/products/X3.shtml) spring to mind, though these types of boxes have fallen out of favour and there are far less available than there used to be.
(You've not gave much detail about the venue/size/capacity etc, but three X3 a side would give you a VERY controlled 60 degree system that would sound phenomenal and give you great bang for buck. The SPL from the fully hornloaded high sensitivity system is prestigious and I'm willing to bet you'd have to pay a LOT more to get the equivalent SPL from line array cabs.)
Smaller narrow dispersion boxes like Arcs or JM-1P have great pattern control at the top end with their HF horns, and while the reflex loaded woofer has little pattern control on its own other than the natural roll off as mentioned, once you stick a few together in an array the width of the sources (just like the dipole of the LA cabs) starts to provide that for you.
At the end of the day it comes down to the same thing rental companies and installers deal with all the time. Do you spec the system that is absolutely best for the room sound wise, or are you forced to take other considerations into account; in this case do the acts or entertainment expect and specify particular brands or set ups, and do they have enough swaying power that their wishes take priority.
K