I think if you try to look at it in terms of "realism" within the terms of the game world it does end up hard to tell why it's that way - though I think trying to look for "realism" in a game set in the 3300s doesn't work: think of what you see as a metaphor for something as far outside our experience as the internet, space flight, and so on would be for a 7th century peasant [1]. It makes for a more exciting and comprehensible game than the life of an AI subroutine responsible for optimising thruster performance on a remotely-piloted ship.
In gameplay terms, trade goods, bounties and exploration data are "potential" - and if you die, you don't keep them, similar to how in the previous games if you died between stations and reloaded your save, you didn't get to keep the beneficial consequences of your failure. Assets, on the other hand, get kept - your ship and credits get kept when you reload the save after death (well, mostly kept - the insurance fee is new). Data and Materials are Assets in this model, intended to be accumulated over a period of days or months - instead of goods and bounties largely intended to be cashed in next time you dock - so they're not lost on ship destruction.
This model breaks down in a couple of places.
1) Exploration data: because there's no major risk while exploring and the incremental risk is pretty low, and there's no time limit in the way that ammo or fuel will eventually force someone out of a RES, or cargo capacity limits a trader, pirate or minor, you can accumulate ridiculous amounts of data. Going on the DWE, for instance (depending a bit on exploration style), could easily get someone from Aimless to Elite in a single trip, which is not remotely plausible for other professions. The original exploration designs had extremely strict limits on endurance, for which the losses would be much lower and perhaps more comparable to "lose a T-9 of Palladium" - still something to be avoided, but by the time you're risking that much, you've probably gained the experience to usually survive it.
2) Engineer cargo (Nanobreakers, for instance) feels more "assety" than "potential" to a lot of people, but I definitely think it's deliberate on Frontier's part that Engineers take a mix of "asset" and "potential" items as payment.
(It'll be interesting to see, if storage gets implemented, if they then switch data/materials on board your ship to the "potential" model. There are arguments for and against)
[1] Said peasant, by the way, would find it very implausible that 21st century personal transports can't be directed to get you home while drunk, because the horses of their time already do that, and is complaining down the local inn about how unrealistic Ye Olde Elite Dangyrouse is.