Its pretty simple as Sneeky pointed out it comes down to noise level.
Take a look at say an H100i V2 noise levels at maximum performance even with 2 fans which spin at 2400 or so RPM.
When I tested the H105 with similar fans it hit 54 dBA when offering maximum performance. the H150i Pro hits just 42 dBA with fans spinning at 1600 RPM
The H110i being a 280mm based unit with 140mm fans simply moves more air as it has fans spinning at 2100 RPM.
Simple fact higher RPM = more noise but also greater airflow. Going by Corsairs own specifications the H110i has fans that push 113 CFM meanwhile the H150i has fans that only push 47 CFM 47 x 3 vs 113 x 2 The 280 mm based H110 has a theoretical max airflow of 226 CFM vs the H150i at 141 CFM.
Essentially the reason for the small 280mm based units superior performance is simply greater airflow. These Asetek based units really don't scale beyond 240 / 280mm.
Corsair has taken a design that typically for 120mm based regularly hit 50+ dBA and dropped it down to 42 dBA. Such a huge drop in noise from an 800-1000 RPM loss on the fans maximum RPM means performance will of course suffer. However no more leaf blower noise either.
The thing is, H115i PRO has two much quieter 140 mm fans on a 280 rad at much lower price making H150i PRO pointless UNLESS you go for the maximum performance with high RPM fans or 280 rad does not fit your case.
Cheap crappy SP fans vs Magnetic Levitation bearings. The higher cost is due to better quality fans. No if you dont need them or RGB the older designs are a better value. However those that would generally change the fans to Noctua or something else. Well thats no longer necessary. The ML fans are what is driving up cost.