Lobbyists influence important policymakers, including members of Congress and the White House. With this in mind, such individuals shouldn’t be able to hide convictions for serious crimes which may indicate corruption or a broader unsuitability to serve as advisors.
As a professionalization of activism, lobbying is very tricky to regulate without overstepping First Amendment protections. It’d be better to risk the occasional lobbyist hiding past convictions, than to risk curtailing free speech rights in order to reveal a small number of individuals.