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In Year 6 Harry uses the notes scribbled by The Half-Blood Prince to become top student in potions.
As we later find out that the Half-Blood Prince would go on to become the Potions Master at Hogwarts, why would they continue to use text books that didn't include the better instructions?
Two points here: 1. Magic is not science, where the textbooks may be out of date by the time they are published. 2. Even in science you will have professors who love the textbook they used fifty years ago and make their students use the same. I have had it inflicted upon me to use and be tested on material from textbooks otherwise long out of print.
Which is not as unreasonable as it seems at first glance. I have what is widely acknowledged to be THE NMR book but has been out of print since the 80's. Does that make it inaccurate or unworthy of reading? Nope, just hard to get. But if you do, it's the clearest explanation of all of the involved science. Similarly THE book on TEM only recently was updated to a second edition but I believe that it too was a good 15-20 years between updates.
Especially at a pre-college level, and especially for introductory texts, and perhaps for something like potions which are analogous to chemistry, there just isn't going to be much change over a few decades. The big changes will be emphasis of some material over others or integrating, say, chemistry with biology (or the equivalent).
I have not real the Potter books but I have suffered through the first two or three movies. The potions class seems more a lab-lecture than really covering any theory which means their textbook is likely to be little more than a lab manual. Which means there is even less incentive to update.
Professor Snape thinks very highly of potion making and was not about to make it easy on the students.
"You are here to learn the subtle science and exact art of potionmaking... I don't expect you will really understand the beauty of the softly simmering cauldron with its shimmering fumes, the delicate power of liquids that creep through human veins, bewitching the mind, ensnaring the senses.... I can teach you how to bottle fame, brew glory, even stopper death -- if you aren't as big a bunch of dunderheads as I usually have to teach."
Remember he was in Slytherin. "Those cunning folk use any means, To achieve their ends." I imagine that his abilities in potionmaking, being something that set him apart from everyone else, are not something he would really want to share with the rest of the wizarding world, let alone a bunch of "dunderheads", as it would diminish his own prowess and recognition in the wizarding world.
A couple factors are in play here:
First, the shortcomings of the book may not have been discerned. If only a small number of students get the simple potions wrong, then the published recipes for those books will be perceived as correct.
Take, on the other hand, a potion whose published recipe is incorrect. Some of the students, while thinking they are following the instructions, actually do something very slightly differently, and wind up succeeding. One of two things could happen here: Either the potion gains a reputation as being very difficult, or the community concludes that the recipe is incorrect. The former can easily become the prevailing opinion, even though it is quite wrong.
Then along comes Snape, who has such a gift for potions that his notes were sufficient to turn Harry from excellent to outstanding. He's finding all of these small places where the published recipe is just slightly off, and his corrections fix them.
Second, getting the establishment to change its mind is a process that is as much political as it is factual. Real-life history has more than a few examples of people who come up with an innovation that is rejected because it does not jibe with conventional wisdom, and only much later is it finally admitted that this "maverick" was right all along.
If as Potions Master he had found a way to stop unloading his issues onto his students, he could very well have become Hogwart's most popular teacher, and he would have been in a very good position to challenge the orthodoxy and get the books corrected.
Unfortunately, he was a bullying piece of work.
