A lot of stuff typically come down to supply chain constraints. The iPad Pro likely sells in vastly smaller quantities than a MBA, and considering that the M4 chip is probably produced in limited quantities at first, it makes sense to introduce it in a lower-volume product like the iPad Pro.Apple showed us their priorities when they put the M4 into an iPad before a Mac.
And for someone like myself who so happens to be in the market for a new iPad, but whose MBA is still going strong, the inclusion of the M4 chip in the new iPad could not have come at a more apt time as well.