Cost of capital. If you have an expensive cargo load on sea for 80 days port to port, it's a lot cheaper to finance than for 120 or 150 days. Same reason we air-freight some cargo today, rather than sea, even if it's not perishable. Interest rates for this business were also higher than the risk-free rate, and rates were higher overall.
“But in the middle of the 19th century, demand for fresh tea was such that the first vessel home from Fuzhou or Shanghai could command a premium of at least 10 percent for her wares, and a clipper ship that cost perhaps £12,000 or £15,000 to build might bring home a cargo worth almost £3,000 on her first voyage.“
