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There are several reasons including: - budget limitation(s) - staff contract(s) expire - stagnation of the particular series - the story resolves itself in that length of time - Gundam isn't a fantasy shonen adventure (like Dragon Ball Z or One Piece) where the story follows the same hero(es) for 500+ episodes/volumes/chapters on conveniently infinitely appearing adventures - stagnation of the particular series

If you count Seed and Seed Destiny together, they would exceed 50 episodes by far.

Otherwise, approx. 50 episodes max is the standard format for a full-length Gundam series.