Mango pickle is made from hard green fruit that would be inedible eaten raw. The unripe stage is chosen because ripening removes exactly the properties the pickle depends on.
Firmness comes from intact pectin
An unripe mango is held together by pectin in its cell walls, which is what makes it hard enough to cut into pieces that keep their shape.
Ripening activates enzymes that break that pectin down, softening the flesh. A ripe mango cut into a pickle jar would disintegrate into pulp within days.
Texture is a large part of what mango pickle is, so structural integrity through months of storage is a requirement rather than a preference.
Acidity is highest before ripening
Green mango is strongly acidic, and that acidity is consumed during ripening as the fruit converts acids and starches into sugars.
Low pH inhibits many spoilage organisms, so the fruit arrives already contributing to its own preservation. A ripe, sweet mango would offer sugar to microbes instead of suppressing them.
The sourness is also the flavor the pickle is built around, since salt, chile and oil are all working against an acidic base rather than a sweet one.
Starch matters more than sugar here
Unripe fruit stores its energy as starch, which does not ferment readily and does not feed yeasts the way free sugars do.
That is why a sweet pickle made from ripe fruit needs a much higher sugar concentration or a cooking step to be stable, while a sour green mango pickle relies on salt, acid and oil.
The distinction explains why the same fruit produces two entirely different preserved products depending on when it is picked.
Variety is chosen for the purpose
Some mango varieties are grown specifically for pickling, selected for a firm flesh and a sharp sour profile rather than for eating quality when ripe.
Using a dessert variety picked green gives a workable but less characteristic result, since the flesh tends to be softer and less aggressively sour even at the unripe stage.
Sourcing green mango in the United States
Indian, Latin American and Southeast Asian grocery stores stock green mangoes seasonally, and they are sold explicitly as cooking fruit rather than as underripe eating mangoes.
Any home preserving project should follow published guidance from a recognized food preservation authority on acidity, salt levels and jar handling, since preservation safety depends on conditions rather than on following a family recipe by memory.