MetaMask makes it easy for new users to get started with Ethereum partly because it doesn't require users to download the full blockchain to start using Ethereum-enabled sites.
We do this by hosting blockchain servers for free, via a great service called Infura.
Sometimes, a site can load a lot of data over our servers, and if enough users use that site, it can affect our service for users of other sites. At times like this, we are forced to rate limit, or even blacklist sites from MetaMask compatibility.
Our first incident of this sort was the EOS Token Distribution site. Their site loaded a lot of information off the blockchain, and it made MetaMask unreliable for users around the world.
If you find a site has been rate-limited or blacklisted from MetaMask, you can get around this limitation by running a local node.