Great question. The answer boils down to (1) the methods of Spanish colonization in Florida focusing on missions rather than settler colonialism, and (2) the disastrous influence of English slaving raids emanating out of the Carolinas.
Early Spanish attempts at settler colonialism in Florida were a comedy of errors. In 1521 Juan Ponce de Leon’s second journey to Florida ended in quickly after landing on the Gulf Coast. Calusas attacked his party, wounding de Leon with an arrow. The entrada returned to Cuba, where de Leon died of his wounds. In 1526 Lucas de Ayllón mortgaged his fortune to mobilize a group of 600 colonists to head toward the U.S. southeast. He established San Miguel de Gualdape, the first Spanish settlement in what is now the United States. The colonists arrived too late in the season to plant, and fell ill, likely due to contaminated water sources. After Ayllón succumbed to illness, the colony fractured and abandoned San Miguel. Less than 150 colonists survived to limp back to Hispaniola. After losing an eye fighting Cortés at Cempoala in Mexico, Pánfilo de Narváez was appointed adelantado of Florida. His unfortunate decision to split his land and sea forces after landing near Tampa Bay was but one of many disastrous mistakes. Hunger, hostilities with the Apalachee, and illness diminished the strength of the land forces, who failed to reconnect and resupply with their sea-based comrades. Narváez decided to skirt the gulf coast back to Mexico, and died on a make-shift raft blown out into the Gulf of Mexico near Galveston, Texas. Only four men, including the famous Alvar Cabeza de Vaca, survived the final overland journey through Texas and into northern Mexico. Finally, Hernando de Soto survived the conquest of Peru, only to die on the banks of the Mississippi after pillaging his way through Florida and the greater southeast. The exact location of his grave remains unknown and the tattered remnants of his forces limped south to the Gulf of Mexico.
After the excesses of cruelty seen in the initial years of contact, the 1573 Comprehensive Royal Orders for New Discoveries, placed missionaries at the forefront of exploration and pacification of new lands. Instead of valiant conquistadores rampaging into new territory, the Orders prohibited the entry of unlicensed entradas into new lands, and prohibited violence against Indians, under threat of fine or death. Entradas were to be led by conquistadores of the spirit, most often by the Franciscans along the northern frontier of the Empire, though Jesuits briefly established missions in Florida, as well as longer-lived missions among the Pima in southern Arizona.
The frontier missions obvious purpose was to provide a spiritual harvest for the Catholic faith, supported financially by the Spanish Crown. A secondary role was to provide an economically expensive, but vital, frontier presence against encroachment from other European nations, and protect interests further south. New Mexico and Texas provided a supportive buffer for the lucrative mining enterprises in Northern Mexico, and Florida provided a safe haven and support for ships crossing the Atlantic. Over the course of the 17th century, New Mexico cost the crown 2,390,000 pesos, while Florida cost three times as much. The colonies were worth the expense as it was thought missionaries could pacify lands at less cost, and with more lasting impact, than soldiers, or soldiers supporting settlers.
The Franciscans arrived in Florida in 1573, and one hundred years later a string of missions, organized into four mission provinces, stretched up the Georgia coast and along the northern frontier of Florida, into the panhandle. Though covering a wide geographic range, the pure number of missionaries operating in Florida was small and usually hovered at slightly less than 50 friars. In 1655 70 Franciscans ministered to an estimated 26,000 converts in the Florida missions.
Indigenous slavery existed in the Americas prior to contact, and European slavers were often the first people to make contact in the Americas, advancing far ahead of officially sanctioned entradas. Unofficial traders and fisherman plied the Atlantic and Gulf Coast, and they routinely augmented their stores with unwary captives, either for sale as slaves in the Caribbean or Europe, or to serve as translators for later voyages. For example, during the first official entrada to Florida in 1513, the Spanish encountered Native American populations along the coast that already understood a few words of Spanish, and fled from the new arrivals, leading Juan Ponce de León to assume slaving raids preceded him. The slaving raids from Carolina were something altogether new.
The English in the Carolinas used slaving raids as a tool of war against Spanish Florida, as well as a means of raising capital. Traders employed Native American allies, like the Savannah, to raid their neighbors for sale, and groups like the Kussoe who refused to raid were ruthlessly attacked. When the Westo, previously English allies who raided extensively for slaves, outlived their usefulness they were likewise enslaved. As English influence grew the choice of slave raid or be slaved extended raiding parties west across the Appalachians, and onto the Spanish mission doorsteps. Slavery became a tool of war, and the English attempts to rout the Spanish from Florida included enslaving their allied mission populations. Slaving raids nearly depopulated the Florida peninsula as refugees fled south in hopes of finding safe haven on ships bound for Spanish-controlled Cuba.
Accurate numbers will be hard to come by for this period. The best we have are estimates, in many cases provided by the Spanish fathers and secular authorities who watched as Florida was overrun by slavers allied with the English. Gallay believes 4,000 Florida Indians were captured and enslaved between 1704 and 1706. In 1708 the Governor of Florida, Francisco de Corcoles y Martinez estimated ten to twelve thousand Indians were taken from Florida. Father Joseph Bullones reported that four-fifths of the Christian Indians remaining in Florida after 1704 were killed or enslaved. The scale of raiding was so catastrophic that refugees fled south, hoping for transport and safe haven in Cuba. A ship captain carried 270 Florida refugees to Cuba in 1711, and said he left 2,000 Christian Indians and 6,000 more seeking baptism when he departed the Florida Keys. Gallay's very conservative estimate for the total number of people enslaved, not counting those who died in the associated warfare and displacement, in Florida alone is 15,000-20,000. The peninsula was practically depopulated of Indians by the early eighteenth century.
So, the answer is a little complex. The Spanish influence in Florida was more explicitly geared toward missions, and less focused on the settler colonialism seen in other borderland outposts like New Mexico and Texas. This left fewer chances for admixture and creation of a new Hispanic identity in Florida. While one might have developed, given time, the English slaving raids of the early 1700s forced the abandonment of the missions and swept away indigenous allies. As Spain's influence in Florida waned, the peninsula was slowly repopulated with indigenous refugees fleeing slavers throughout the U.S. Southeast, Lower Creeks pushing south, and escaped African slaves. The Seminole Nation emerged from this loose confederacy of refugees, and grew to formidable strength during the 1700s.
Source cited:
Gallay Indian Slave Trade: The Rise of the English Empire in the American South, 1670-1717