Why does the family have to tell everyone when they take a bath in "The House on Mango Street?"

In Chapter One, the narrator tells us about the house she hoped she would have. She imagines it would be large and beautiful, and that it's the house her Papa talked about and the one her Mama dreamed about.

She says:

"And we'd have a basement and at least three washrooms so when we took a bath we wouldn't have to tell everybody."

But then, the narrator describes the house that her family actually does move into, and it's nothing at all like she hoped it would be. Instead of being clean and white, it's got crumbling bricks and a "swollen" front door. There's only one bathroom, which everyone has to share. With the narrator, her mom and dad, and "Carlos and Kiki" and "Nenny," that makes six people sharing one bathroom--so if you're going to be in it for a little while, taking a bath, you'd better tell everyone first in case they need to go in there and use the toilet soon.

Your question was "Why does the family have to tell everyone when they take a bath in 'The House on Mango Street?'" and the answer wasn't directly written in the text: that is, even if you read the whole chapter carefully, you won't find one sentence that tells you something like, "They have to tell everyone when they take a bath because..." It's one of those questions that requires you to understand what happened in the chapter, and figure out (infer) the answer on your own based on what you read.

The question might be important to consider because it shows us the difference between the narrator's expectations and reality. She wanted a wonderful house with more than one bathroom, but she got an old and cramped house with a single bathroom. That contrast sets the mood for the whole book: we see hopes vs. reality, and out of that contrast we see the narrator rising above the disappointment and setting her sights on something better, like when she says at the end of the chapter:

"I knew then I had to have a house. A real house."