Flavour gets bad, and if you disassemble it and look at the wires you can see crud accumulating.
Different juices will kill coils at different rates, as will the coil quality. Sweeter juices especially premium american flavours can kill a coil in a day, but get clean juice and a good coil and itll last months.
Tbh most prebiult coils go really ott with wicking, if you get good at building your own the difference is night and day in terms of flavour and longevity. Plus when building your own you can just re-wick after dry burning the crud off the coil so its not like you need to twist new wire every time.
I tend to find when building that spaced (gaps between the coils) last longer, and dont require nearly as much faffing to get even glow.