Most likely about surface area, heat distribution, and oxygen content. Fire needs fuel and oxygen to burn, and there’s more oxygen on the side (above and below) than the middle (just below). Paper needs to be at 452 degrees farenheit to catch fire. It’s easier to catch the corner on fire because you’re focusing on a certain point, where in the middle the heat is being distributed.
A flame needs a balance of three things in order to start. Oxygen, heat, and a fuel source (paper). If you have too much or too little off any of the three though, your fuel will burn much slower or not at all.
In the case of trying to light a paper from the center. You’ve only got one surface for the flame to light, and on top of that it’s a large surface so heat will spread throughout the paper and just warm it up before it lights on fire.
On the corner you’ve got three surfaces in contact with flame and only one direction for the heat to go.