The limitations can be done in two ways:
1)Quantative — when a max number of participants is set and no one is allowed to join after that.
Example: Max 1000 entrants in a facebook campaign
2)Qualitative — when the requirements are raised up by the bounty managers.
Example: Minimum 500 followers to participate in an article/blogging campaign.
Unless they are not a scam project. There are few bounties that requires a limited amount of participants. The primary goal of bounties is to grow their community and have a wider range of broadcasting information about their project. Conducting a bounty is an additional marketing strategy for the project. Usually, limiting the number of participants is according to the rules of the bounty manager and not primarily from the company behind the project itself.
I join some bounties with limited number of participants, some are quite successful and some failed. And yeah, they give a nice amount.