In order to understand the difference, you need to know how a loudspeaker is tuned. A hifi loudspeaker has bare drivers on a flat baffle. There is nothing affecting the sound between the diaphragm and a microphone. A loudspeaker designer than has a full arsenal of tools to affect it. It's like working in a chemical lab, where everything is in clear glass with no contamination and all the tools to precisely dose chemicals. If you want to alter a driver, you make a precise virtual model in software and the software spits out it's response. You know for sure that this response will be very close to the real thing. You can for example put the tweeter in a waveguide and an app will help you iterate the design 20 times before you can get all the lines parallel. You don't need to make the thing 20 times. You're done with the baffle and the thing is still not flat? That's OK, you can compensate any problems in the electric domain. You open another program that lets you make a filter out of electronic parts. It simulates a notch filter here and a shelf there and with 20 parts you have finally linearized the response. The whole thing weighs a ton and barely fits in the enclosure, but hey it sounds great. Oh did I mention that AI does that last part for you? Yup, there is an app that does just that. You give it a measurement and it spits out a circuit diagram.

Now headphones... Remember when I said that you can design a waveguide? So with headphones you are already forced to work with one. The ear cup is a waveguide, a terrible one and there's not much you can do about it. Headphone's ear cup is like room that cannot be detached from the loudspeaker. You can't take it to an anechoic chamber. You need to work with the mess that it creates. Sound is like light except everything is a mirror. A lightbulb in a torch illuminates the surroundings differently than a lightbulb in a chandelier. A headphone is a lightbulb in a crumpled tin can. The result is a mess.

Headphones are tuned by covering holes on the driver with lossy materials. You poke a hole and see what happens. Each time you change it, something drastic happens, but it's difficult to understand what. I have personally no idea what tools or software do the big guys like Sennheiser have to aid them with it. I guess a headphone designer would shed some light on it, not me. But even if you get the driver tuning right, you change the earpads and it sounds differently. More than that. You put it on someone with curly hair and glasses and that makes a difference as well :D That's because the enclosure is lossy. Very lossy. I personally don't understand why we can't add passive filters to headphones. It used to be problem of source output impedance, but now that everything is measured and reviewed, we can predict how a filter would sound. So it would be cool to see filter PCBs for headphones. It won't fix all issues because the most appalling resonances in headphones are destructive, meaning that they cannot be fixed without physical change. It can be used to attack broad valleys and bumps though.