Bioplastics and other emerging eco-industries are very new in comparison to the petroleum based plastic industry which has been around at a global scale since the 1960's.
Eco-responsible industries in general use newer technologies and use higher quality materials so the cost of manufacturing items is higher, right now. Bioplastics only account for 1% of the global plastics market so it has enormous room to grow (and it is). As eco-responsible items become more normalized and more in demand the costs will also level out because it will no longer be a speciality item.
The other reality is that regular plastics are made cheap and dirty from the byproducts of the oil and gas industry. While those manufacturing processes don't cost much they have a devastatingly high carbon footprint cost. More than half of the emissions created by regular plastics come from the sourcing & production of the materials.  From a greenhouse gas emissions perspective regular plastics are doing the most damage before they are even a usable product. When we invest in products that are made better but cost slightly more we can consider this a small investment in a cleaner future.