It could be from not wanting to ruin Riku and Takuto's relationship just yet. They've been together for years, so it might not be as easy for Riku to just stab Takuto from behind and take Shizuku away even if she wanted it; in fact, if she broke up Takuto, Riku might even side with Takuto instead because dancing a wedding over his friend's corpse doesn't give a good aftertaste. There's also issue of her public image (kicking away her boyfriend to date his best friend), which even if Shizuku doesn't mind, would be another deterrent factor for Riku to accept her.
What she wanted is prepare it until Riku has no reason to hesitate anymore.
Not sure about Takuto, but there are hints that he already knew Shizuku's real feeling. This and the previous chapter did hints a lot he has a kind of inferiority complex towards Riku, or at least about how he felt left out (Riku and Shizuku always being naturally for each other, while he had to spend lots of efforts to keep up).