Because they represent Carlton officially as a club and they probably wouldn't like to be represented in that way.
It's a deliberate attempt to belittle and intimidate, making it unnecessarily personal when an umpire is simply performing their job. It's just wrong and improper for a club-sanctioned group calling umpires w***ers in that manner. God knows we may have no umpires left in 10-20 years time, who would want to enter a profession like that? Seen enough abuse at local footy matches toward junior umpires to know the toll it can have later down the line.
Do you think the abuse towards umpires has gotten worse over the past 10-20 years? How long have you followed football? Because I can tell you that the abuse used to be much more intense and that it obviously hasn't had the effects you claim.