Primarily because MODIS has 36 spectral bands, though this isn't necessarily an improvement if you have no use for them (AVHRR covers all the vital ones in the visible and infrared). MODIS is also a better spatial resolution, generally at 500m and as good as 250m compared with AVHRR which is 1+km.

Average Sea Surface Temperature 1985-1997 (1 month - AVHRR) 1985-12-01 00:00:00.0-1997-12-31 23:59:59.0.

A. C. Dilley has written:

'Improved AVHRR data navigation using automated land feature recognition to correct a satellite orbital model' -- subject(s): Mathematical models, Radiometers, Artificial satellites in remote sensing, Remote sensing

Jeff Key has written:

'Ice surface retrieval from AVHRR, ATSR and passive microwave satellite data' -- subject(s): Antarctic regions, Satellite observation, Remote sensing, Surface temperature, Algorithms, Arctic regions, Sea ice