They release new editions just to make it harder to use the old ones
That's right. Your 2018 edition textbook you bought for $120 is basically the same as the 2016 version. While there are certain exceptions for classes based around current events or medicine, most are created just to limit the market for used books, where they do not make money. This may be well known, but as one student said: "I'm using an older edition of my chem book right and the only thing that is different is the pictures and some of the page numbers (some pictures added, others taken out)."
Only four publishers control the majority of the market
I am pretty sure I learned about monopolies from one of their textbooks. McGraw-Hill, Pearson, Cengage, and Wiley are responsible for 80% of the books in circulation. This control of the market has only driven prices up and students are paying the price.
Access codes were created to counter rentals
That is exactly why you could not purchase a used book. I was told that I needed an online access code that comes with a new book, and why I paid $300 for my French book this year. The big companies caught on to the used book frenzy and found that they could create these online resources, pair them with the book, and force you to shell out hundreds of dollars just to do your online homework.
History books eliminate unpopular information because it is bad for business
It is the age-old story. We all know our history has been censored in some form, but this is immensely popular within the pages of textbooks. Look, telling middle schoolers that our country set up Japanese Internment camps does not sell. Dipping anywhere below the surface of slavery does not sell. Going in-depth about the treatment of native Americans does not sell. It is natural to want to hide the bad, and many do not want to buy the textbooks for their kids to hear it, thus whitewashing history.