People are incentivized to work on nodes higher in the tree because they have a higher probability of success, that success happens more quickly on average. These two assertions are just the contrapositive* of Nodes deeper in the tech tree take more time and work to unlock.
If you needed more incentive to work on leaf nodes than the fact that you are more likely to win and that victory will come relatively quickly, there are two other points:
How does this play out in the real world?