In the semi-democratic Empire known as the Aurean Dominate, the small peninsula of Sparteia has seen a population explosion over the past two centuries or so. Located in Argentolia, the most populous (and 2nd largest by land area, around the size of the continental US) province of the Aurean Dominate, the Sparteia peninsula (around the size of Greece) was, for most of its history, little more than a stopover point for ships making voyages between the Capitol to the southwest and the gold-rich mountains of Monsaltu to the east. Now, 4 of the 10 largest cities in Argentolia are in Sparteia. What would be a good reason for a mass migration to such a region like this to occur?
Some background on Sparteia:
The peninsula is topographically extremely flat and marshy in the north, to the point where around 31% of the peninsula is considered wetlands. However, the Sparteian Alps begin to rise in the center and start running down the peninsula, making the south's terrain much more rugged. Generally, the east coast of Sparteia has a tropical rainforest climate while the west coast has a drier tropical savanna climate due to the Sparteian Alps' rain shadow. The north has a tropical monsoon climate transitional between the two. Some of the higher elevations in the Sparteian Alps experience a temperate Mediterranean climate. Like the rest of Argentolia's tropical north, Sparteia experiences the Pontic Monsoon in the slighyly cooler months from April-August, in which torrential downpours from the Pontic Ocean to the north contribute the vast majority of the area's annual rainfall. For example, Nirossos Bay on the east coast of Sparteia receives an average of 395 mm of rainfall in the month of June alone, while receiving only 72.6 mm in the warmest month of January.
While these monsoons are very useful for agriculture in other parts of Argentolia's north, Sparteia is almost entirely composed of limestone and has thin, chalky soils that have greatly limited agriculture in the region. Additionally, much of what Sparteia has historically grown are cash crops like coffee, rubber, sugarcane, coconuts, and oil palms on large plantations owned by wealthy Aureans who often do not even live in Sparteia. Food crops, mostly consisting of rice, citrus, mangoes, and bananas, are generally grown on a small scale by village-level subsistence farmers. The small portions of the more temperate Sparteian Alps that can be used for agriculture are almost completely used up by similar wealthy plantation owners as in the north, with wine and olive oil plantations dominating the valleys between the peaks.
The rugged and mountainous south of Sparteia is also known for Sparteian Rangers, a culture of semi-nomadic zebra riders dating back to before the Aurean Dominate conquered the region. Most of them are content to peacefully herd protoceratops and psittacosaurus or serve as light cavalry in the Aurean Military, but enough of them turn to banditry and become highwaymen to make traversing the Sparteian Alps somewhat dangerous.
Due to being almost completely surrounded by coral reefs, Sparteia had a sizable seafood industry even prior to the population boom. Ammonite, spiny lobster, goliath grouper, xiphactinus, and giant reef octopus (giant Pacific octopus but tropical) are all local delicacies.
Prior to this population boom, Zanclatis in the north was the only one of Sparteia's cities to host more than 100,000 residents. Now, 7 of Sparteia's cities (Zanclatis, Sozippa, Nirossos Bay, Cape Diosidos, Rizyra, Pheraci Beach, and Porta Burdigala) pass this benchmark, with 4 of them (Zanclatis, Sozippa, Nirossos Bay, and Cape Diosidos) in Argentolia's top 10 largest cities and 3 of them (Zanclatis, Sozippa, and Nirossos Bay) reaching a population of 1 million or more.
The technology level of the Aurean Dominate would most closely resemble that of the early to mid Ottoman Empire, although the dominant culture is much more Greco-Roman inspired but also varies somewhat by region. Sparteia specifically has some additional al-Andalus and Caribbean flair to it in some aspects, with much of the urban architecture being very Mudejar-looking and the cuisine using tropical ingredients like plantains and coconut milk. Little has changed in the Aurean Dominate technologically since before the population boom, thanks in part to the Aurean Senate's insistence on strict isolationist policies.
Koppen climate map of Sparteia (cities w/ >10k residents in black, >100k residents in white, 10 largest cities in Argentolia province in green). The peninsula where Rigona is is part of the Capitoline Peninsulas rather than Sparteia, so ignore it. Rizyra and Lindesos are generally considered part of Sparteia and have experienced similar population growth, although they are outside the core peninsula.