As with kitchens, bathrooms provide a lot of moisture and plumbing cockroaches prefer. Unlike kitchens, however, bathrooms don't contain food, but they do have something else that roaches love: residues from soaps and grooming products, discarded paper tissues, and skin and hair sheddings. Yes, they are known to enjoy even hair and soiled toilet paper.
One good thing about the love of bathrooms and kitchens is that many cockroach species don't travel beyond these two rooms of the house. They have plenty to sustain them and keep out of bedrooms and upper floors in most cases. German cockroaches, the most common and prolific, however, go just about anywhere but also prefer sinks, bathtubs, areas around toilets and behind every possible surface containing water or traces of humidity.
In the case of bathrooms, pull out the big guns of treatment and traps and do it often. Sprays, by the way, are proven least effective, while powders and traps show better results.