Why does the Catholic Church send people to hell by excommunicating them?

It doesn’t. The Church doesn’t have the power to send anyone to hell. Only God can do that. More properly, people who go to hell send themselves there because they choose hell over heaven, themselves over God.

Excommunication isn’t a condemnation to hell. It is the Church’s top judicial sanction and involves excluding a notorious sinner from communion with the faithful. (Someone who is “ex-communicated” is “outside of communion.”) Excommunication is intended as a warning to the sinner that he needs to repent.