The given purine has two structures as shown below.
In $\ce{9H–Purine}$ (figure 2), lone pairs are far apart from each other (3 and 7). Comparatively in $\ce{7H–Purine}$ (figure 1) lone pairs are very near each other (3 and 9) . Nearness of lone pairs in $\ce{7H–Purine}$ could leed to greater repulsions between them. This could make it energetically unfavorable in comparison to $\ce{9H–Purine}$.
This could be the reason why $\ce{9H–Purine}$ is more favored tautomer compared to $\ce{7H–Purine}$.
