Why would an 8-order LPF switched capacitor have a less steep roll-off curve (MAX7405)?
For a lowpass with order n=8 you can expect a magnitude drop (far above the cut-off frequency) of 48dB/Octacve. I think, the presented curve does show such a slope - however, only approximately. Why do you expect a damping of 40 dB at 2 kHz?

More than that, Bessel filters are optimized with respect to their phase response (linear). The price paid for this linearized phase response is a magnitude which exhibits a relatively broad transition between passband and stopband.