This was often out of necessity. For example, the original 3rd Edition Ork codex was released in mid-1999. It remained the "current" codex until a 4th Edition was released in late 2008.
Warhammer Quest likewise.
These days Gorkamorka is only really available in PDF form from enthusiasts; Games Workshop had the PDF's on their site for a while, but not any more.
Dungeons & Dragons 3.5 edition and all prior editions are completely out of print. With 3.5's staggering popularity, prices for used copies of the more common books are slowly on the rise. Wizards of the Coast used to sell PDFs of all of its current products and any TSR products as well, but they were all pulled from websites like RPGNow and DriveThruRPG in Spring 2009.
Every edition has this. Check the prices on eBay for some of the favored old AD&D modules and editions. Check the prices for Planes of Conflict or Menzoberranzan or The Ruins of Myth Drannor or Spelljammer: Adventures in Space. As of 19 Sep 2011, the asking price for some of these sets (the original Ravenloft boxed set or Hellbound: the Blood War) is higher than the retail price for the products when they were new.
In January 2013, WotC opened a DriveThruRPG subsite, "Dungeons & Dragons Classics", and started releasing supplements from every D&D edition, up to and including 4th, with the intent of having hundreds of titles available.
Star Wars d20 RPG articles and modules posted on the internet by publisher Wizards of the Coast were taken offline by the company when the license expired in May of 2010. They're still out there, kept online by fans who saved them before they were taken down.
The Old World of Darkness is quite popular to this day, and though no longer in print still sells briskly on the second hand market and has players who play it today.
White Wolf have started making their back catalogue (oWoD and nWoD) available through print-on-demand, with the intent of having everything available. Oh, and they've started putting out new oWoD PDFs in tandem with nWoD PDFs. They're definitely working on moving the oWoD over to 'Rescued'.
The BattleTech magazine Battletechnology falls into this category as well. Originally published in-house by FASA, it was later "farmed out" and passed through several different publishers before finally ceasing publication, leaving behind a tangled mess of ownership. The result is that it will never be compiled or re-printed. Furthermore, by Word of God, material from Battletechnology will not be used in the Tabletop Game unless it is non-contradictory and the original creator can be tracked down. (Several mechs from the magazine have appeared; in those cases, they were either created in-house to begin with or the writer is now a member of the BattleTech creative staff.)
The original Dune boardgame is considered a classic, but is direly out of print, and going to stay there, due to rights issues.
Two of the first Yu-Gi-Oh! movie cards, namely the "Pyramid of Light" and "Blue eyes Shining Dragon", haven't seen a booster pack/Promotional release, and the only ones existing are the ones who went into the movie, and kept them in good condition, or mostly worn out.