[MisterMuncher]

The thing with CF isn't that it's strong as such. It's incredibly strong in the directions it is designed to handle at the layup phase but fucking marginal *at best* once forces go off-axis. As such, it really, really doesn't like to be clamped, as that's a force in all directions. Bike components will usually have a clamping torque spec of somewhere between 5 and 8NM, which is not a huge amount of force, but when you're applying that force with a typical 4mm Allen key, it's fine, it's just a bit more that snug. For comparison, torque to fit pedals into cranks (metal to metal, even "carbon" cranks have threaded metal inserts) is around 3-5 times that. Once you get out of the designed clamping areas*, things get sketchy very quick. You wouldn't believe how easy and quick it is to write off a £5000 bike frame with a workstand clamp applied to the top tube.

So building a cylinder out of that and subjecting it to the sort of uneven loads and directional tensions involved in the deep sea is fucking idiotic.

*Referred to in the trade as "Black Metal", as the layup has to be overlapped in different directions so much in these areas that it confers no weight or strength advantage over just using aluminum or steel. Carbon handlebar stems are the classic example, there's no sensible reason to ever even try to make a component that will see forces in almost all directions and must have a minimum of four clamping points out of carbon, but people love their bling.

[Malcolm Armsteen]

In motorsport (early adopters) it became clear very quickly that any sort of shock or stress quickly degraded the structural integrity and strength of the material - one example being a crash (John Watson at Monza, I believe) where the car struck the barrier and then spun across the track to hit the barrier on the other side. What had stayed intact in the first impact gave very little in the second (mercifully slower) impact.

This has been known in the sport since the 1980s.

[MisterMuncher]

Indeed. Not unrelated to that is that CF is made from relatively cheap raw materials/precursors and in brute material costs, it's actually much cheaper than metal. The expense comes in with the need for skilled labour in the layup process and manufacturing of moulds for high-quality compaction, sharing and curing. Small-scale carbon will generally forego the mould making process and use vacuum bags in plastic moulds to do it. It's a much less reliable process and whilst cheaper for small numbers of components, much less consistent and more expensive per unit.

In short, it's one of few products where you're going to come out ahead by enlisting a Chinese OEM: they have the tools and the talent (you might need to provide the QC, though).