why lossly compression performance worse than jpeg when compress png？

[jiuzhuanzhuan]

When i compress 24 Kodak pics ，compression ratio even worse than jpeg.
Below is my compression process，

1. convert png to raw by numpy
2. use opj_compress opj_decompress(version==openjpeg-v2.4.0-linux-x86_64)

import numpy as np
from PIL import Image
img = np.array(Image.open('kodim01.png'), np.uint8).tofile('kodim01.raw')
opj_compress -i kodim01.raw -F 512,768,3,8,u -I -r 5  -o kodim01.j2k # set r from 5 to 40
opj_decompres -i kodim01.j2k -o kodim01_recon.raw

When compareed the R-D performance whith JPEG, openjpeg is worse

（bpp means average bits of pixel of 24 Kodak pics, psnr means average Peak Signal to Noise Ratio of 24 Kodak pics）.
So, how to set parameters when lossly compress image with public openjpeg executable correrctly?