Two reasons this usually happens.
1. You’re trying to jump on a generic term that is already established and make it your brand. Ie don’t try claim ‘superfood’ as your brand name and sell fruits and nuts.
2. A much bigger brand exists with the same/similar name. If I make a newsletter platform called chimpmail, aside from being sued Google might think my entity is the same as Mailchimp and not bother adding me to the index.