The likely reason for this is the energy barrier that surrounds Wakanda. Things can't enter the barrier (at least, not without sustaining severe physical trauma), but they also can't exit the barrier. This is demonstrated when one of Thanos' henchmen is propelled into the roof of the barrier by the Hulkbuster's arm, and the barrier slowly incinerates him until he explodes.
If the Wakandans wanted to fire upon the alien ships, they would have had two options:
- Open a hole in the barrier, and fire through it. It seems they
would have been reluctant to do this: when T'Challa orders a hole to
be opened in the barrier to let the aliens through, the other
Wakandans react as though he's gone mad.
- Invent a weapon that could fire through the barrier. The film appears to indicate that they don't have such a weapon, otherwise they would have been firing at the aliens before they passed through the barrier.