Some Norsemen did have long hair indeed. It is expected in a tribal culture to have cultural variations between areas or even tribes. However, long hair was not a normal or a cultural pattern for males during the Viking age, especially those involved in raiding, and thus, combat.
While men invariably wore a beard, they were also well groomed, which typically included shorter hair. A primary reason was the simple fact that long hair was impractical in battle, in the north, in the cold, on long trips, etc... The depiction of Vikings with long hair comes from the Romantic period of the late 18th to mid 19th century, at the same time Vikings were also depicted with wings on their helmets.
Historically, Skjǫldrinn also actually banned long hair over 1,200 years ago, first for Úlfhéðnar (including Blendingar) and extended it to Berserkir a few years later. At the time, the primary reason was that it was unsuitable for combat.
Practical reasons for having shorter hair are numerous. In contrast to a beard which held many benefits.
1. Purpose: Long hair serves no purpose in the field or wilderness environment. Length does not provide extra warmth or protection, and is actually a liability. A beard, on the other hand, has many benefits.
2. Hygiene: Long hair is difficult to keep clean in the field. It requires water and cleaning agents, both of which are extra weight, extra water (in a liquid state), and extra time. In contrast, a reasonably size beards is not only much easier to wash, but gets less dirty. Also, body hair, as in pubes and pits, actually improve hygiene.
3. Wildlife Attractant: Long hair will either smell from natural oil and sweat, or from the products used to wash them. Both of which are attractants for predators such as polar bears and grizzly bears. Not an issue with a beard, which gets less dirty, is more easily washed, and has much less of a smell due to spacing, density, and position of hair.
4. Hypothermia: Long hair requires a lot of water to wash, and requires energy to dry. This means head is exposed to cold during the wash and after the wash, increasing the risk of hypothermia. On the other hand, a beard requires much less exposure to cold during washing and drying, and is also in a part of the body, the face, that is less sensitive to temperature variations and is not an issue for hypothermia.
5. Lice: Long hair is a breeding ground and ecosystem for lice. Which can be spread to the rest of a group/unit. These lice, however, cannot normally thrive and survive in a beard, due to different hair spacing.
6. Safety: Long hair gets entangled in equipment, which can lead to being essentially scalped. Beards don’t because they are, well, right in your face, giving you more control as to where it’s going, they are shorter, and when caught in something, beards don’t result in scalping.
7. Security: Long hair are like handles so your head can be grabbed for the purpose of killing you, including holding you while slitting your throat. Not an issue with a reasonably size beard which is much more difficult to grab due to position, length, and human anatomy.
8. Performance: Long hair get in your field of view, affecting your marksmanship or bow shooting performance. Beards don't.
9. No pheromones in hair on head. So any scent is external, from dirt, and doesn't serve an evolutionary purpose.
10. Look like a man, not a girl: Last but not least, it is very difficult for most dudes to pull long hair without looking like a girl or a bum. Beards, on the other hand, makes ugly guys look better, and good looking guys look awesome. Ultimately, in the Viking age, long hair was the typical feature of a woman, not a man.