In the interest of "no bulli the Sayori," she committed suicide because Monika ramped her depression into overdrive. It's even suggested in Sayori's last "poem" that she put the idea in Sayori's head through speech instead of scripting.
When Sayori is having one of her freakouts she mentions that she should just do 'what she told me to' or something like that. Monika definitely told her to off herself. ---