I don't understand why razz is not more popular. Is it an especially difficult game to learn or play? Or are there other causes that keep the game from becoming popular?
Razz has never been a popular game. Razz is a form of seven card stud. The best hand is a "wheel" which is a 5 high straight (a-2-3-4-5). They also play a 2-7 version. These days in brick and mortar poker rooms, the only time you see razz is when it is included at a mixed games table.
In Nevada, from the 1960's through the mid-late 80's the only regular Razz game was at the Stardust in Las Vegas (AKA the Algiers in the movie Casino). Just like was depicted in the movie, the poker room there had a number of lowlife hustlers working in teams. The razz game was famous for being full of cheating and colluding players. The game was a limit game, typically 15/30 with a dollar ante. Razz may also have been played on a hit and miss basis at tournaments and other rooms. But for the most part the game was considered dirty so most card rooms just didn't promote or spread it.
In the mid-late eighties a few events happened. Organized crime was about done in Las Vegas, so the cheating game of Razz wasn't so protected anymore at the Stardust. The Las Vegas Hilton opened a large poker room of 26 tables in their new super book, and solicited razz players to bring the game over there. By this time the Stardust was way beyond it's heyday (see Swingers), and wasn't attracting very many players of any kind. California opened their poker rooms through a court ruling to Stud and Holdem, and Indian gaming began, and poker started to spread. Atlantic city opened poker rooms about this time. So there was some pressure for Razz players that were serious poker players trying to make a living to move on to greener pastures. (Poker at Wiki)
At the Hilton they reduced the ante in the game by half to fifty cents. This had the effect of cooling the collusion some. But by this time the player pool consisted of mostly aging poker players. Literally as the poker players slowly died off the game died off. Other markets and other games like Holdem and Omaha were also slowly draining some players away, and also capturing any of the new players that came along. For the decade or more before the new markets came along poker was low growth enough in Las Vegas to be virtually stagnant. The Hilton poker room home of the only regular Razz game in the world in the late 80's was slowly loosing business to the new emerging markets. Then in 1989 the boom in Las Vegas started with the opening of Steve Wynn's Mirage. This killed the room at the Hilton and consequently the Razz game.
Razz was occasionally played at the Mirage. But it never became a regularly spread anywhere as near as I could tell. I never overheard players talking about some razz game here or there. When I was running my websites between 98 and 2008, one of which listed about six hundred poker rooms, all of which we surveyed by phone for the types of games they played we never found a regular razz game.
For all practical purposes Razz is a dead game. No regular cash games, only to be found at an occasional tournament, WSOP has a $1500 Razz event still, and for cash almost exclusively just one of the games in a mixed game stack. I have played it online, but that was a decade ago and really don't know if it is offered anywhere in that venue.
So that short history of razz, should answer part of your question, it is not popular now, because it has never been popular.
So lets discuss why it has never been popular, and in my opinion will never be popular and is forever slated to be a niche game.
Of all the variations of poker, Razz is the most simple minded boring game of them all. Its a game of simple ranks of hands, one does not need to consider suites, or straights. Since it is a seven card game, pairs that are bad, are not so bad. One can still make a clean low with two pair, three of a kind, flushes, and straights in their hand. Drawing to a hand is not a bad thing, everyone is drawing. Everyone misses draws and bluffing, mostly desperate bluffing is the norm. These things raise the general frustration level of the game. This simplicity also narrow the EV between the skilled player and the unskilled player.
Like any poker game, Razz is fun to learn, fun to play for awhile, but when you get down into the grind of the game, Razz sucks. So many negative factors all come together to make it suck. With low EV for even the best of players things get desperate and the players gravitate toward cheating. The players get nasty because the ones that keep coming back to the game are the worst sort of degenerate, lowlife addicted gamblers, or successful cheats. Not even the worlds best Razz player stands a chance against the cheats. And it does not take a great player to get as good as you can get at Razz. It also does not take a great mind to cheat as well as one can cheat in the game.
Razz will always remain a niche game, because when it is spread on a regular basis the game degenerates into a really dirty game to play more so then any other game. That's the bottom line as to why it has never been popular and never will be.