Why are harbour line trains so dirty?

I use the harbour line train every day to commute to work but I feel every day it just keeps on deteriorating. It is so dirty (attached a pic and sorry for that). I have been noticing this since a lot of days and decided to just post today. I have travelled using western line train also for initial few months and they are so good. I really want to know how often do these trains get cleaned and how often overall maintenance happens. Please don’t say it is the crowd :(