Big waves and high winds will cause your vessel to go unstable and crash. Ocean-crossing cargo vessels have to handle all weather conditions.

Hydrofoils significantly increase draft and nobody wants to dredge their ports and canals 20 feet deeper.

Also drag is proportional to velocity squared, so you can save a ton of fuel just by slowing down a bit. Much easier than designing a hydrofoil cargo ship.

Mikael at Candela here. Because it's super hard to make the control system for an inherently unstable boat. It took five years of R&D to make it work in all manners of situations, banked turns etc. Conventional boat builders don't have access to software and control systems engineers, and tech companies seldom invest in the marien sector. But once we got it working, there are no real drawbacks, only benefits. Smooth, quiet, and fun to drive. Check out candela.com for more videos.

This is anecdotal but in the past ~5 years I've noticed foils popping up in more and more places (in Oahu).

One mistake going too shallow and you will probably rip the foils off doing significant damage to boat. Thats my theory why they aren't more popular except in specific applications. Fuel costs are always an issue with boats and foils are well known to reduce them along with smoother ride.

Most normal pleasure boats regularly go in shallow waters and pull up on the beach etc. If you look at the skegs of any of these boats you will see where they have dragged or struck something and a normal outboard doesn't stick down anywhere near as far.

It's cool the the Candela can retract the foils, but again forget to do it one time and those foils looks very expensive and what do they do to the hull as they are being ripped off.