Thank you. That makes a lot of sense.Welcome to Beesource, Sharon!
Assuming you mean "why" do bees sometimes abscond from open screened bottom hives, most likely its a combination of factors.
First, lets note that there are some partisans that argue that open screened bottoms are not the problem, its just a coincidence.
In my opinion, new packages are less of a cohesive unit than an established hive. Package worker bees are not necessarily from the same original colony (before they were put in the package). They certainly don't 'know' their queen, except for the time in the package.
And, as noted by some posters earlier in this thread, the open bottom allows more light and airflow in the hive than in hives with a solid bottom (or a box with the screen closed off). Bees heavily rely on pheromones for communication, and excessive airflow may disrupt/impede such communication. Open screen bottoms impede the bees ability to thermo-regulate their brood nest.
So, in my opinion, its a number of individually (possibly small) factors, but when added together some new packages just decide that the conditions are not right, and decide to 'seek their fortune' elsewhere.