Why are retroviruses such as hiv notoriously difficult to treat?

Mutating&Evolving: It was difficult to treat in eightees when the virus was discovered&there was no drugs to treat but over the years many new drugs and combination of drugs have been discovered&it has become easier to treat Yes it is difficult to treat because retro virus is constantly mutating,changing and basically evolving and that's the reason the virus evades the drugsThats why it is difficult to make Vaccine