Question

Why Can`t the mystery gift last forever?

Answer 1

Because gamefreak wanted it that way
The entire reason for event Pokemon is that they are RARE! If you could get them anytime anywhere there would be no point in having events. Besides that would lower the trading value of any Pokemon that was an event.

Comments

you can't trade event pokemon anyway ya know xD

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I was able to trade my keldeo to my sister once.

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Correction, PX. You cannot trade them over GTS. I was perfectly able to trade my Keldeo with the infared.

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I mean't Wi-Fi, i never tried it via infared

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Correction number 2, PX. It's Infared, not WiFi. The two things ain't same :3