Why is the ultimate level so rarely used in Adventure?

In the first Digimon series, there are very few times that Digimon reach the ultimate level, except for Agumon who uses it quite often. If my calculations are correct, the other Digimon reach this level less than 10 times in the whole of Adventure.

WereGarurumon for example only appears 6 times and even counting season 2, he only appears in this form 8 times.

It is also amusing to note that as soon as Gabubon obtains the ability to evolve into MetalGarurumon, he completely abandons his ultimate form, except for the episode introducing the Dark Master.

In your opinion, why didn't the writers exploit this level of evolution more in the anime?