The basic strength of aniline is less than aliphatic amines due to:
Resonance in aniline delocalizes the nitrogen lone pair into the benzene ring, reducing basicity compared to aliphatic amines, as per the PDF.
Ref: NCERT Class 12 Chemistry > Chapter 13: Amines > Topic: Reaction with Nitrous Acid and Diazonium Salts - Part 1