Because eventually perigee coincides with summer or winter in either the Northern or Southern hemisphere (and then the apogee will coincide with the other hemisphere's opposite season, naturally), so one of them will get more sunlight (heat). BUT! The Northern hemisphere has much more landmass than the Southern one, which accentuates the differences between seasons because land heats up faster in the summer and cools down faster than the oceans in the winter, and also land provides a platform for ice accumulation. This is the reason that glacial periods involve much more ice in the Northern hemisphere than in the Southern hemisphere.
It is generally believed that glacial periods set in when the Northern summer coincides with the furthest point from the Sun on Earth's orbit, while interglacials set in when the Northern summer coincides with the point of closest approach to the Sun. Summer is the key because once ice lasts past the summer then summer cannot heat up the Northern hemisphere as much as usual: ice reflects a lot of the Sun's energy. Conversely, if the summer is warm enough to melt more snow and ice than was accumulated during winter, then you can't have a glacial period.
It is important to note that there are other orbital cycles that matter here, especially the precession of the equinoxes, which is the cycle that relates which hemisphere gets its summer closest to the Sun. Earth's axis of rotation itself rotates, very very slowly. The precession of the equinoxes is a 25ky cycle.
Another cycle is the angle of the Earth's axis of rotation to the ecliptic. This varies a fair bit over the years, and this one is (IIRC) the most chaotic of the orbital cycles.
All of these cycles, known as the Mylankovitch cycles[0], have winter/summer difference accentuation/attenuation effects. The Mylankovitch cycle theory is that when these coincide in certain ways you get glacial or interglacial periods.
It is generally believed that glacial periods set in when the Northern summer coincides with the furthest point from the Sun on Earth's orbit, while interglacials set in when the Northern summer coincides with the point of closest approach to the Sun. Summer is the key because once ice lasts past the summer then summer cannot heat up the Northern hemisphere as much as usual: ice reflects a lot of the Sun's energy. Conversely, if the summer is warm enough to melt more snow and ice than was accumulated during winter, then you can't have a glacial period.
It is important to note that there are other orbital cycles that matter here, especially the precession of the equinoxes, which is the cycle that relates which hemisphere gets its summer closest to the Sun. Earth's axis of rotation itself rotates, very very slowly. The precession of the equinoxes is a 25ky cycle.
Another cycle is the angle of the Earth's axis of rotation to the ecliptic. This varies a fair bit over the years, and this one is (IIRC) the most chaotic of the orbital cycles.
All of these cycles, known as the Mylankovitch cycles[0], have winter/summer difference accentuation/attenuation effects. The Mylankovitch cycle theory is that when these coincide in certain ways you get glacial or interglacial periods.
[0] https://en.wikipedia.org/wiki/Milankovitch_cycles