@Aeriandra
From what I understand, the unwritten policy of one scanlation group per series is based on keeping with the quality and consistency of releases, so that sudden name changes aren’t implemented (and if they are, they’re at least explained and/or edited into previous releases by the group to reflect that change). Likewise, the quality might not be on par when someone outside the main scanlation team uploads their own version.
This tends to anger the usual scanlation team because they feel like all their current work on the chapter that has been just released by someone else is now meaningless. All the more so when scanlator bashing appears when a newer group/person releases a chapter faster than the first group scanlating the same series, and make it appear that they are lagging behind. The cause for this delay could be for any reason, from real life issues to group tactics in the practice of releasing multiple chapters at once rather than the usual pace of a single chapter.
Another reason is, as
@SirCuddleschmidt pointed out, it's not uncommon for smaller groups to burnout faster than larger scanlation teams.
Meanwhile, some groups are as @AbyssalMonkey described and intentionally prevent other scanlation groups from picking up the series that are under their group name. Teams that intentionally withhold releases though, are generally despised by the community at large.
Other times, their intentions are not as malicious as the above and they simply overburden themselves by taking on multiple projects that it is difficult for them to adhere to a regular release schedule. Typically when this happens, the series being handled by the group take different priorities and are consequently affected by the team’s own interest (either lack of it or not) and/or the commentary by readers (based on whether or not the group is interested or not). This tends to lead to a lot of infrequent updates on certain series.
Ultimately the other team can still release their own version, but some groups don’t because they appear repetitious and/or feel it is meaningless to upload a second version (unless they feel theirs is significantly superior).
That all said, I strongly agree that no one owns the right to scanlate a particular series (unless the author has given their express approval) and am of the same opinion as what you have written on the issue of monopolization as well as the original purpose of scanlation, that is, to share the series with a broad audience that is unable to access the original -- I think you clearly expressed how I felt on the matter.