A procedure call is explicit, a trigger is implicit. You don't call a trigger, it just happens as a side effect of something else. People tend to forget implicit things. Suddenly you notice that something is acting strangely or slowly in your application. You can look at your functions and procedures and try to find the problem. But if your application has triggers all over the place, how do you know what is going on? A trigger can change a dozen rows, which in turn can change other rows, so changing a single row can fire thousands or millions of triggers. Also, triggers are not fired in a particular order, the database is free to change the query plan according to what it thinks is best at the moment, so triggers are not deterministic. Triggers can sometimes work and sometimes not.
Almost everything that can be done with a trigger can be done with a procedure, but explicitly, deterministically and in most cases without side effects.