You should consider the question the other way around - why are the UV/vis absorptions of aggregated (bundled) nanotubes reduced? This is common for aggregates of pi - conjugated molecules. The vibrational modes (as measured by Raman) are similarly quenched, and the process can be used to monitor the dispersion process -see attached.

Segregated nanotubes in the colloidal dispersion it means absorb a characteristic wavelength, 260 nm attributed to the nano size and thereby a shaper peak aggregates of various sizes which will not only shift the characteristic wavelength of absorption to right but also over a host of wavelengths which may give a broad absorption band and not a sharp peak.