The behavior of decreasing refractive index n for wavelengths greater than 700nm is an important result, because it corresponds transmission increasing of ITO /glass at near-infrared wavelength
the reflectivity (here vertical incidence) is given by R = ((n2 - n1)/(n2+n1))2.
Here n2 and n1 describe the refractive indices at the boundary of the 2 media 1 and 2 for the corresponding wavelength. If the difference of the refractive indices decreases, decreases R and T increases. A low reflectivity means a high transmittance (the absorption is described by the imaginary part of the refractive index and must be discussed independently).
If the question was, why n decreases with decreasing wavelength, the resonances which can occur must be discussed for the corresponding material.
I'm not an expert in ITO, however an increasing transmitance generally means the decreasing refractive index. I would expect that in ITO (as a wide-gap semiconductor) the normal dispersion will take part. Consequently, at near infrared wavelengths the free-carrier absorption will occur.