Why Cointreau contains bitter and sweet orange?

Sweet orange peels and essences give to Cointreau their juicy, fruity orange aromatic notes, as well as a bit of floral such as orange blossom, rose, and lavender.
The sweet orange (Citrus sinensis) peels and essences used in Cointreau come from the Sevilla region in Spain, Ghana, Senegal, and Brazil. The major varieties are Cadenera, Salustiana, Pera, Late Valencia, Sweet Mediterranean, Washington, and a local Ghanaian variety.

Bitter orange peels and essences give Cointreau a layered bouquet: initially fresh like min or very zesty lime, then spicy bergamot, and finally pepper and cardamom notes.
Bitter orange peels and essences (Citrus auriantium ssp bigaradia), better known as Bigarade oranges today, come from Brazil and Tunisia. This orange’s leaf and flowers are also appreciated by the great “nez” of perfumery. 

Those sweet and orange peels are magnified with just three other ingredients: pure water, sugar and neutral alcohol.