Hi and thanks for your question. I'm not sure if there is one main reason as there are a number of reasons that you brought up. One is that the staff has to wash their hands and the skin of the patient well, before inserting the catheter. The longer it stays in the more chance you have of getting an infection as bacteria build up in the bag and around the insertion site. The bag has to be lower than the bladder. Sometimes they have remove it and clean and reinsert. They have to empty the bag often. Sometimes a kink the line can cause a problem. Also, Staphylococci like plastic and can adhere to plastic and they are common on the skin. So there are lots of reasons why you can get a UTI when having a catheter. But I would say unclean practice of inserting or leaving it too long would be the biggest reasons. Sometimes with the elderly or immuno-compromised, there is nothing you can do to prevent this.
Hope this helps,