Probably some projects have limit as to how many members they can accommodate on a telegram or discord channel. But it doesn't make sense because they want to spread the news of their project to as many as they can.
But perhaps they don't want to be taken advantage as well with their airdops and cannot participate anyone.
Anyhow, not sure if airdrops is still worth this 2022 though, just asking because it seems that the airdrop hype is gone and projects are no longer giving participants good amount of it. Or the value is not worth anymore as compare to like 2017 or early 2018.
Worth it or not actually depends on the project being worked on. If the airdrop is held by a crypto project that has good potential going forward, it will certainly give quite good results. Although it is not easy to get points. Because most airdrops give a few points for one task performed.
To get lots of points, you usually use a referral link, the more people who join the referral link, the more points you get.
But regarding telegram groups that limit members, usually because the member quota is full. The member limit for telegram groups is 200,000 Members. I have several times found groups with 200,000 members and even created two groups to accommodate these members.