Why were there not more aliens at the time of the fireworks?

First, the film indicates that there are only three creatures in the area. The whiteboard in the basement has a note that the family has spotted three different creatures, and after the gunshot you can see two more running towards the house. It's not covered in the film if this is due to the creatures having established territories & packs, the result of a geographical boundary, or some other reason.

As for why the gunshot drew them straight in but the fireworks did not, they are very different sounds. The gunshot makes a loud noise in one spot inside the house, whereas the fireworks created many different sounds from multiple moving objects over a long period of time, and with the loudest sounds actually occurring in the air above the farm. While all the noise certainly drew them to the general area, it would have been nearly impossible to track the sounds of individual people over all the noise of the fireworks.

A bit of unconfirmed speculation: during the fireworks, the other two may have been attacking nearby homes. In a previous scene, the father lights a fire on top of the grain silo and notes several other fires being lit in response nearby - presumably a prearranged system for nearby survivors to communicate that they're still OK. After the fireworks, the children briefly light a fire on the silo, and do not see any fires in response. Maybe it was the wrong time to light fire (no one watching that late), maybe the fire wasn't lit long enough to be seen, maybe everyone is still in hiding at the other communities after hearing the fireworks, or maybe things went very poorly for the other survivors during the fireworks...