How come French is by far the most distinctive of the major Romance languages?

[Tooting Carmen]

Compared to Portuguese, Spanish, Italian and even to an extent Romanian, French is far more distinctive in terms of vocabulary and in particular pronunciation. Why is this? What are the historical reasons behind it?

[Tietar]

Compared to Portuguese, Spanish, Italian and even to an extent Romanian, French is far more distinctive in terms of vocabulary and in particular pronunciation. Why is this? What are the historical reasons behind it?

no, Spanish only has 5 vowels, Latin and the other languages have many. in addition the J/G and C/Z of Spanish also has its exclusive pronunciation. French has nothing special

[Ouistreham]

no, Spanish only has 5 vowels, Latin and the other languages have many.

No. Latin also had only 5 vowels, like Spanish.
(Which is the reason why Latin alphabet is poorly adapted to the pronunciation of languages with 15 vowels sounds or more.
Hence the use of diacritics and the notoriously erratic spelling of French and English.)

However those 5 Latin vowels were distinguished by length, with generally a more open sound for E and O (this distinction has been maintained in Italian, not in Spanish).
Long stressed Latin vowels became diphtongs in Spanish (porta —> puerta).

French is far more distinctive in terms of vocabulary and in particular pronunciation. Why is this? What are the historical reasons behind it?

Celtic substrate? Probably not. The Gaulish language died out around the 5th Century, just before Old French began to emerge.
Germanic influence: obviously, especially as for the tendency to reduce all root words to monosyllables, the use of front closed vowels, and non-pro-drop verbs (an explicit subject is always mandatory), these 3 phenomenons being intercorrelated.

Then, some special features developed following the inner logic of the language. In most French dialects, initial hard K sound became a soft sh-sound (castellum —> château), nasal vowels appeared, and word stress was lost.
That's the way every language evolves. Starting from a common basis, Spanish and Portuguese (or Danish and Swedish) developed highly diverging phonetical systems.