Why do you think Atwood chooses to reveal the rebellion of Moira through the character of Janine in Chapter 22?

Janine has just had her baby; the women have all been there to help her. Because Janine was privy to Moira's escape, she is an ally in her own way........... the women tend to look down on the handmaidens, coo over their babies, and call them whores as they walk away.

I think Moira's rebellion is revealed here because it was the 'women's time,' and nobody else was allowed in their midst. Offred is now alone and free to ruminate over her escape, to realize she'd envied her closest friend her freedom. Janine's giving birth has sparked Offred's memories. After all, Moira was the one to slap Janine when she was hysterical. Moira had the common sense and the tenacity to actually pull off the escape. Moira's escape also gives the other women a sense of hope; for awhile.

"Moira was like an elevator with open sides. She made us dizzy. Already we were losing the taste for freedom, already we were finding these walls secure. In the upper reaches of the atmosphere you'd come apart, you'd vaporize, there would be no pressure holding you together. (Offred, 22.45-47)

All of this has been taken away with the women's final knowledge that Moira hasn't really escaped.......... their hope is really lost. Remembering Moira's story gave Offred hope and made her life a bit more bearable........ that feeling left the instant she'd found out Moira hadn't really escaped and was working at Jezebel's. Even Moira had been beaten.