It is very common for mission territory to be evangelized by religious orders; up until the end of Communist rule in Yugoslavia, for instance, only Franciscan friars were allowed to minister to Catholics in that country, so all parishes in Croatia and Slovenia (which are Catholic majority) were staffed by Franciscans. After that, a secular Bishop has been installed and secular Clergy are being implanted, and this sometimes causes friction with the pre-existing Franciscans. This is one of the reasons for the reported "dislike" of the Bishop of Mostar-Duvno for the apparitions at Medjugorje.
Regarding the Colonial Americas, the mission in the New World was mostly given out to religious orders: the Jesuits kept mostly to the Portuguese Americas and the Plata river valley, Dominicans took care of the Andes and the Caribbean, and Franciscans evangelised in Mexico and what is now the West/Southwest of the United States. Until the 19th Century or so, there were few dioceses and the Bishops had precious few secular priests to minister to the faithful concentrated in the colonial capitals like Salvador, Buenos Aires, Lima, San José, México and New Orleans. The ministering to the indians and their evangelisation out in the country required special training (even linguistic), which the Orders were usually best at providing.
Eventually, as the lands were evangelised and colonised, the Orders passed the baton to the secular priests, as they changed their focus to other underEvangelised places like Africa and Southeast Asia. Obviously, the "Enlightened Despotism" of the mid-18th Century saw the Orders have much of their possessions confiscated by the secular governments of Portugal and Spain — the Jesuits were even suppressed by the Pope at the insistence of the Marquis of Pombal, Portugal's prime minister — and the secular priesthood stepped up to fill the void.
Finally, regarding Canon Law, there is nothing in it that forbids Orders from having possessions like monasteries and associated land; it's more or less the point for Cloistered orders like the Benedictines, and the famous Jesuit and Franciscan missions were also organised in a like manner.