Because BlockChain.info is the most widely used bitcoin wallet.
This wallet is easy to make, does not require special tools, can be accessed from anywhere simply by using a web browser. Its features are quite advanced.
Because this wallet is the most widely used.
Just because it's the most widely used does not mean that it is the safest. blockchain.info is, with its security, still a web wallet. I use Electrum as it is feature-full, but I recommend buying a Trezor or Ledger hardware wallet, because of its security: the privkeys never leave the hardware device. The good thing is that you can use Electrum with a Trezor or a Ledger, and this allows you to have the security of a hardware wallet, and at the same time, take advantage of Electrum's features: the ultimate combination.