This is a good question that must come to many people's minds when they see the two very similarly sized (Taiwan only slightly larger) islands. The similarities are a even more numerous than the visual.
- They were both long at the margins of power in Chinese empires,
- had significant minorities who vigorously defended their autonomy (in Hainan it was the Li),
- they both served as major pirate bases
- and as refuge for those fleeing political turmoil on the mainland
- they both saw major economic transformations under Japanese occupation (though former during only a few years)
- both today have a complex multiple layers of demography and dialect thanks to successive but well spaced out waves of immigration.
However, Taiwan has had far more strategic and economic importance, and Hainan only grows somewhat in importance in the 20th century.
This is not because of the reasons you suggested: lack of harbors or natural resources. Hainan, though seen as an "island at the end of the world" [6:229], and a "backward place of the Chinese empire" where exiles were condemned to go [4:385], it was close and accessible to the mainland (24km, vs 144km for Taiwan), has 68 harbors [4:391], and was a "treasure island" (宝岛) with some 30 important minerals [1:10] (especially iron ore that was most fully exploited under Japanese occupation [5:101]), in addition to being a good place for rubber (60% of all Chinese rubber production at some point [1:12]) and sugar production.
I would argue that the main reasons for the relative greater attractiveness of Taiwan to various powers, at least up to the 20th century comes from its location:
- Taiwan is very close to the Okinawan islands. Yonaguni only 100km away, and Philippine islands around the same distance from SE islands off Taiwan [3:23]
- Mongols saw it as a great launch pad for invasion of Japan, though failed to take the island in 1290s [3:37]
- The Sino-Japanese pirates thrived particularly well in Taiwan since it was at the center of important regional trade routes right between Japan, China, Philippines - which made pacification a greater priority for mainland powers [3:49-50] and others alike.
- The Dutch wanted in on Fujian trade action to tie up with West Java, but they most wanted access to profitable Xiamen just across the water and Taiwan was a kind of "runner-up" gained with acquiescence of contenting powers.
- For Koxinga and others who sought refuge - and thereby became a target, Taiwan was a much better launching pad, than marginal Hainan in the far south
- Lands across from Hainan were poor and relatively underdeveloped so not ideal as colonial outpost. Zhanjiang (Fort-Bayard) across from Hainan didn't see much development, even under French control from 1898. Europeans wanted the much better territory near trade rich Guangzhou (e.g. Macau and Hong Kong), or further up coast in Xiamen etc. As a result first stable European presence not until Jesuit mission to Hainan in 1632, and later a few Danes and US missionaries in the 19th century but only some 10-12 foreigners reported in Hainan in early 1880s [1:11]
Other factors may include the relatively stable and early migration/control by the Chinese into Hainan (despite limited control over the Li in the mountainous areas) in 110BC [4:390], vs. first notice of Taiwan by Chinese imperial records in 230AD [3:35] with migration much much later. Early control and administration made Hainan less vulnerable to invasions. Hainan also fell under French "sphere of influence" in late 19th century which was recognized by treaties with the Qing and Japan, and perhaps saved it from the early 20th century turmoil (up to Japanese invasion) [5:93-95].
This situation all changes in the 20th century. The Japanese saw Hainan's naval importance for its southern advance [5:93] and kept it under full direct control instead of turning it over to Wang Jingwei's administration [5:99]. This is again true after WWII, as Hainan was among last territories to fall to the Communists with the massive naval attack in spring 1950, and later Hainan becomes a strategically vital area in tensions between China and Vietnam (west coast of Hainan has many naval basis) and as administratively connected to China's claims on the potentially petroleum rich reserves in the South China Seas [4:167]
Notes above done as: [Source Number:Page Number]
Kjeld Erik Brødsgaard, Hainan: State, Society, and Business in a Chinese Province (Abingdon, Oxon; New York, NY: Routledge, 2009).
Richard Louis Edmonds, “‘Hainan Province’ and Its Impact on the Geography of China,” Geography 74, no. 2 (April 1, 1989): 165–169.
Jonathan Manthorpe, Forbidden Nation: A History of Taiwan (New York: Palgrave Macmillan, 2005).
Chaio-min Hsieh and Gong-fu Zhong, “Hainan - the Island of South Sea A New Province in China,” GeoJournal 20, no. 4 (April 1, 1990): 385–391.
R. T. Phillips, “The Japanese Occupation of Hainan,” Modern Asian Studies 14, no. 1 (January 1, 1980): 93–109.
Anne Cseste "Ethnicity, Conflict, and the State in the Early to Mid-Qing: The Hainan Highlands, 1644–1800" in Pamela Kyle Crossley, Helen F. Siu, and Donald S. Sutton, Empire At the Margins: Culture, Ethnicity, and Frontier In Early Modern China (University of California Press, 2006).