Why the Stigmata?

In my last entry I mentioned the feast of the Stigmata of St. Francis and that leads me to ask, why is it on the Roman calendar? I could understand having St. Peter's Chains or Chair, St. Paul's Conversion and St. John before the Latin Gate, since they were the chief apostles and in their writings laid the foundation of Catholic theology but why the Stigmata of St. Francis?

I suppose the immediate answer is that the sanctity of St. Francis, work of his order and popularity of his cultus merited it. However another explanation could be that this feast, which I am sure was first commemorated only by the Franciscans, was introduced into the Roman calendar in recognition of their role in the development and promotion of the Roman Rite, since in humility they accepted the rite of the universal Church. However this is only speculation, so if readers have information on why this feast was included in the universal calendar they would be most welcome.

PS Apologies to any Franciscans, be they members of the Order or simply lovers of Il Poverello.