Scott's point (lighter compounds boil off first in vapor-feed mode) is correct, but is the smaller factor, typically.
Often, even when your upright canister starts off well above the boiling point of the fuel, the following happens: At first, with a warmish canister, there is sufficient pressure due to the fuel boiling in the canister. But WHENEVER and WHERE-EVER a liquid changes to a gas, it absorbs heat from its surroundings. This is how sweating cools us. It is why more volatile alcohol cools us off more quickly if spilled on our skin. If the liquid to vapor transition is in the canister, the canister is cooled. With little to be gained from the cold environment, the canister temperature can fall to near the boiling point and the flame goes to almost nothing. Not a problem with on a hot day or with a higher-pressure fuel (although in some of my non-BPing pursuits, I vaporize propane so fast that it can form ice on the outside even on a warm day, and eventually lack sufficient pressure.
However, in an inverted, liquid-feed canister, the liquid to vapor transition ISN'T in the canister - it's in tubing near the heat of the stove - sometimes, like in a white-gas Coleman stove, right in the flame. That provides the needed heat and leaves the canister at its original temperature. You need the canister to be warm enough to start and in cold climates you need to keep it from cooling due to snow, cold air, etc. But the canister isn't rapidly cooling itself by vaporizing liquid into gas. And therefore, inverted canister stoves are good to a significantly lower temperature and requires less elaborate schemes to reflect stove heat onto them, etc.
In a pinch, a tea candle under a canister will warm it up pretty quickly and provide the pressure needed to run a stove. Never let the canister get more than warm to the touch.
With upright, vapor-feed, you need to keep putting heat into the canister. That can be from a warm day, heat reflected from the stove, a pot of warm water, or the candle scheme. What constitutes a "warm day" varies with different fuels as others have noted.