One reason (or the main reason) to level sit in the low or mid levels is so you can find plenty of people to fight. This appeals to people who want lots of action, want to become famous, or want the opportunity to rack up edge points through pk they would have a tough time getting as a hero. It is easier to find assassination victims or potential charges for your unholy in the pk-rich middle range as well.

Importantly, the low and mid levels have fresh people coming through them regularly and low level characters have fewer hitpoints, so the advantage goes to whoever has a better set and surprise. At high levels, people are not as easy to catch off guard, there are usually fewer people in your range, people are less likely to go from 100% to 0% in the span of one lagging manuever, and after a few fights, people know what tactics the aggressor is likely to use and will counter accordingly.

Other reasons are to practice skills like assassin kicks, invoker spells, or find wands, but code changes have gone a great way in mitigating these as practical reasons.