Because the lone pair of electrons of double-bonded nitrogen is unhybridized that is why this nitrogen is more readily protonated.

The lone pair of electrons in the single-bonded nitrogen is involved in aromatic conjugation, therefore, the double-bonded nitrogen is more easily protonated.

Due to less steric hindrance, the double-bonded nitrogen is more readily protonated.

Because the lone pair of electrons of single-bonded nitrogen is sp³ hybridized that is why this nitrogen is not readily protonated.