Part of her becoming so bold was because she started becoming afraid of losing Tomoya when she realized what was happening. But yeah, there's a story written taking place after the end of Kyou's route where it's brought up that Kyou doesn't really move as fast with Tomoya as Ryou did, or something like that. The tsundere is too strong in her, I suppose.
And yeah, Kyou also didn't go for Tomoya because she was afraid of screwing up their existing friendship, etc etc. She figured that if Ryou and Tomoya went out, Ryou would be happy, Kyou could still be near Tomoya, everyone's a winner.
Didn't quite go as planned.