I need to know why does FreeStyle Libre lasts 14 days and no longer than that
It's programmed to cut out after 14 days and that will be partly a business decision, but another reason is that the sensor filament has an enzyme called glucose oxidase on it which oxidizes passing glucose, breaking it down into other chemicals, the breakdown gives off electrons which are measured as an electronic current which is then turned into a reading: more glucose, more electrons, more current, higher reading.
Over time the glucose oxidase wears out and the sensor will start to fail. We know that Dexcom users can get much more out of a sensor than the official 7 days, sometimes 2, 3, even 4 times as much and they then notice the readings start to get noticeably more erratic and noisy.
Part of that will be the glucose oxidase wearing out and also "biofouling" - the filament basically gets clagged up with biological detritus so the glucose can't actually get to the filament to he measured.
No one has yet been able to hack libre to get more time out of it (xDrip+ will get an extra 12 hours but that's as far as it goes).
Based on what we've seen with the dexcom hacks, it's pretty clear that a libre sensor could run for longer if someone found a way, but Abbot has likely done some number juggling and decided, not least to get approval, 14 days is a safe bet to (a) keep profit levels up and (b) stop it before the oxidase wearing out and the biofouling starts giving sketchy numbers.
It's 14 days here but in America it is (or was, not sure if it's changed now) 10 days - they wanted FDA approval for bolusing from it so they had to cut it to 10 days. I expect that was because the FDA was saying after 10 days, it gets shakier.
