There are several reasons:

1) There's no way to go from a Solar Eclipse to a Lunar Eclipse using nothing but Moonfire. You will lose a Solar Eclipse via SnF spam, but afterwards be stack because SnF will revert back to MF and not move the Eclipse bar.

2) Eclipse is actually more powerful than the damage increase from Lunar Shower. Lunar Shower makes your initial hit of MF deal more damage, but it doesn't increase the damage of the DoT ticks. The DoT ticks are far more valuable. Also, remaining in Solar to have Eclipsed Insect Swarm and Wild Mushroom -- which is better single target damage than SnF spam -- is more valuable than the damage increase you get on MF.

3) Cycling through Eclipse at 8 Energy per cast takes forever. You'll cast MF 25 times to go from one end to another, and half of those won't be Eclipsed. Plus, 8 is a very wonky number in relation to the energy gains of our other spells, so there are cut offs where the energy gain doesn't actually reduce the number of casts needed to reach the next Eclipse.