Pretty much what it says in the title. Other events had omni-directional beam array/wide-arc dual heavy cannon awards, like this season's Herald antiproton set, and rare sets have been moved in and out of purchasable access before so it can't be an issue of giving up a profit incentive. Why limit this particular option in this way?
Events that include weapon packs in addition to the main reward of a ship, have always been limited to Beam Arrays, Dual Beam Banks, Cannons, Turrets, Dual Cannons, and Dual Heavy Cannons.
Comments
This was true for each of the following events:
The Herald weaponry set we got from the Demon Hunting event, gave out those specific weapons only.
If you want Omni/Wide-Arc weapons, you usually have to go about collecting them the regular way - buy from the exchange, or open some lockboxes.