Two reasons(that are sort of related):

a. Decolonize

'Punjab' is a colonized rendition of the word. The British spelt 'Punjab' as such to pronounce the 'u' as a long vowel, such as the 'u' in the word 'lure.'  This distastefully and indolently delineates from the native pronunciation.

When the British annexed Panjab in the mid-1800s, Panjab's literacy rate was exemplary. In villages, it was compulsory for all women to study qaide, or language-learning books, so that they themselves would be able to read and write while supporting their children in doing the same. In its height, the Sikh Empire was steadily progressing in academics and the arts. In order to dismantle the thriving spirit of the Panjabi people, the British plotted to nip their growth in the bud. British forces are known to have burned qaide in homes to ensure that Panjabis did not have the knowledge keys that would nourish their advancement as an independent people.

Spelling Panjab with an 'a' instead of a 'u' reclaims it from the bounds of colonialism.

b. Semantic Precision

In my book, I have developed a didactic system that transliterates Panjabi into Latin letters so that those who are not familiar with Gurmukhi, but have familiarity with the Latin script, can still read Panjabi. 'Panjabi' adheres to this transliteration system more accurately than 'Punjabi.'