The only real answer is that we don't know, and while can try to identify some drivers, there's no real "Magic Formula" of reasons which explains it perfectly.

Generally, it'll have something to do with institutions. Tuscany managed to achieve an institutional balance of power both inside and outside of its cities: the inclusive institutions common in Italian cities, which fostered trade, prosperity, and freed up surplus resources for patronage of the arts and culture, survived for a surprisingly long time in Tuscany (the Medici would only formally dismantle Florence's republican institutions in the 16th century, three centuries after the Visconti had done the same thing in Milan, for example). In addition, in in spite of periodic conflicts between cities in the region, Florence's ascendance to dominance was actually completed rapidly only in the 15th century: There was no "slow-burning" expansion as was achieved by Milan, Verona, and even Rome, which tied up resources for them. The balance of power in tuscany allowed for resources to remain free to be spent on artistic and cultural endeavors, be it art, literature, philosophy, or what have you.

But there’s always an additional "Why," especially because the answer, "It's Institutions" sounds a lot like a shrug saying, "Because They Just Did." Unfortunately, identifying more drivers is difficult, especially in our case here because it's not like the rest of Italy was disinterested in the arts: artistic communities emerged in Lombardy, the Veneto, and many other parts of the peninsula. And we can't even really say if tuscans objectively surpassed other regions of Italy in artistic production, only that they certainly were recognized by people in other parts of peninsula (and Europe) as being particularly productive in the arts. So if anything, we should look for reasons that Tuscany's republican institutions fostered the export of artistic trends, matching the more general tuscan trend of exporting high-value-added goods. Additionally, we can add that Tuscany was particularly so longevous in its output, even if it’s unclear if this was cause or consequence. At any rate, in the 13th centuries it seems that Sicily was the most artistically productive part of the Italian region, benefitting as it did from a sumptuous courtly life, but not only was there limited success in exporting this output (except, perhaps unsurprisingly, to Tuscany) this output also petered out with the decline of the Southern monarchy. Why did Tuscany sustain its artistic primacy for so many more centuries?

We can dabble in some careful geographic determinism: Tuscany, generally speaking, is a hummocky landscape unsuited for intensive agriculture. This means that the region actually had difficulty supporting large cities, and where they did emerge, large cities could be serious siphons of resources from the hinterland. This only made it difficult for these urban councils to project decisive power (thus a surprisingly longevous balance of power emerged between Siena, Pisa, and Florence) but it also created a general scarcity of labor. In other words, labor was expensive in Tuscany, especially compared to the the much more agriculturally productive northern plain (as expected, it was also easier for the larger northern cities to project power: Milan and Verona were able to build large areas of geographic dominance long before Florence did. Interestingly, Venice similarly only acquired an extensive landward area of control in the 15th century, and like Florence very prosperous and artistically productive, albeit less successful as an “art exporter”).

Here we can move into the (possibly) more solid ground of Economic History: Scarcity of labor means wages tend to be higher, and while the cost of goods should generally match wages, in tuscany a steady influx of goods (especially agricultural goods) thanks strong trade links with the rest of Italy and wider Europe seems to have kept prices from rising too much. Scarce labor and high wages also puts pressure on productivity: unable to scale via intensive agriculture, tuscan capital owners were attracted to high-value added activities deriving from what the terretory could support, notably pastoralism (like the wool trade) or cash crops (like wine; Pisa's loss of dominance in the Tyrrhenian Sea is partially attributable of having to alternate manpower between the galleys and the wine harvest, while their competitor Genoa had no such manpower constraints).

On the one hand, this means that the region's economic and political leadership was pushed towards urban institutions and into mercantile and commercial activity, and proximity as well as participation in urban institutions (notably urban councils) created a desire for intangible prestige-enhancing activity like patronage of the arts. The wide circulation of high-value-added activity also means that standard of living was higher in Tuscany, especially for urban professionals who were not only more numerous, but could earn more, work less, and had more free time to devote themselves to things like poetry, literature and philosophy, and also had more free resources to spend on hiring artists and craftsmen to embellish their living environment, contributing to a regional culture of appreciation and patronage of the arts. Let's also not forget the longevity of tuscan cultural output also matched the longevity of its republican institutions, meaning that individuals with a propensity for philosophy or political science could strive to be entrusted with positions of responsibility in government, also contributing to a culture fostering expenditure of time and resources on knowledge and culture.

But the issue with geographic determinism as the main driver of the institutions which fostered prosperity and the arts is that it can't explain why regions with similar characteristics didn't develop the same way. Across the Apennine Mountains, cities with similar cultural background and surrounded by equally hummocky central-Italian landscapes like Ancona and Urbino, while prosperous and beneficiaries of respectable artistic communities, did not match their tuscan counterparts in both economic and cultural output. Likewise, it cannot explain why by the 16th century, Rome and Venice overtook Florence as the peninsula's cultural epicenter (just as an example, while Tuscan remained the written standard for literature, most dictionaries and grammar primers were actually published in Venice by the time the printing press came along). So we always end up looping back to institutions: Tuscany's republican institutions were more inclusive than those in Urbino, and its trade links stronger than those in Ancona, but also didn’t adapt to the new geopolitical landscape which emerged in the 16th century (neither did Venice and Rome really, but all that matters here is that they did a better job than Florence at any rate).

In the end, we can go on as long as we'd like listing as many drivers as we want which which could have contributed to tuscan artistic output. All we know for certain is that Tuscany's institutions were inclusive, its cities were locked in a balance of power, and its standard of living was high. This much we can attribute to driving the region’s ability to produce and export art and culture.