The brand was well known prior to let's encrypt. They also offer flexibility that is less risky, e.g on longer duration certificates.
There is also strong assurance it works across old browsers, old computers and across the world.
Additionally you can do multi domain validation and wildcards too. There is also Extended Validation where they actually check your business exists, etc and no cases of it being faked, etc.
There is also a perceived higher standard in the quality of assurance offered as it is a for profit enterprise and if there was a breach for example it would hurt far worse than say for Lets Encrypt.
