Usually because the campaign is for a different audience than your profile, or because the network it runs on isn't connected yet. Here's what each case means.
"Not Eligible To Apply"
The brand chose who the campaign is for, such as which countries, and your profile doesn't match. The campaign is open, just not to you. Check that the country and gender in your "Profile" settings are right, and look for other campaigns under "Eligible to apply" on your Campaigns page.
You need to connect an account
Each campaign runs on one network. If yours isn't connected, you'll be asked to connect it while applying ("You must connect to apply"). Connect it under "Social Accounts" in your settings.
"Campaign Not Yet Open"
The brand hasn't opened applications yet. Check back later.
The campaign has closed
Once a campaign stops accepting applications, the deadline can't be extended. Look for other open campaigns.
You've already applied
You can apply once per campaign. If your contract for a campaign was cancelled or expired, you can't apply to it again.
Fewer than 1,000 followers doesn't stop you applying. You'll just see a note that it may affect your selection chances.