Answer:

This question has been addressed before, but it's one that's worth discussion again occasionally.

There are many reasons why anime series are as long as they are, and these reasons may vary from specific to very arbitrary. Furthermore, to really understand the relative length of particular anime series, one needs to be aware of a bit of the background of the series as well. First and foremost, anime is a commercial product, so series that make money get lots of episodes, and series that don't turn a profit are usually quickly laid to rest. Series that aren't expected to have a broad appeal, and shows adapted from relatively short manga series may also tend to be shorter than average. Some series are created specifically for a target audience that will only support an estimated given number of episodes. And finally, sometimes series may be affected by circumstances and the commitments of the production staff.

Anime fandom in Japan is not quite the same as it is in the west, and some series that are very popular in America are not well known or particularly popular in Japan. For example, although "Battle Angel" is a perennial favorite in America, the series was ended after only two anime episodes because Japanese fans did not take to the show and Japanese sales were disappointing. Other titles that are popular in America including Fairy Princess Ren, Hunter Lime, Dragoon, Gestalt and Dragon Half were only mildly successful in Japan, and not big enough hits to justify continued production of more episodes.

Many short anime TV series have the number of episodes that they do because of their intended broadcast time-slot. With the sheer amount of anime broadcast on Japanese television, it's not uncommon for Japanese fans to loose interest in shows after a while and move on to a different show. If a title is perceived to already have a limited viewing audience to begin with, it's sound financial sense to make only 12 or 13 episodes and broadcast the series late at night for half of a 26 week Japanese TV season. Series like Basara and Hellsing may seem like they have cliffhanger endings suited to more episodes, but these series were intended from the beginning to air at 1 in the morning for only half a season. In the case of a series like Vandread, which proved to be unusually popular, a second season appeared less than a year after the conclusion of the first. The AD Police television series was such a show that aired late at night and was only intended to be a half-season series. In fact, the AD Police TV series would never have been made at all if it were not for the financial assistance provided by AD Vision as a co-executive producer.

In some cases, series are created in response to a certain demand, which predetermines their length. The Petshop of Horrors television series, for example, was specifically intended to have only 4 episodes because it was created to be a special television series broadcast for one month only- one episode per week- during the variety program Wonderful. Short series like Adventures of Kotetsu, Angel Sanctuary and the first Sakura Wars OAV series were created specifically to appeal to Japanese fans who were already familiar with the manga and video games these series were based on. Angel Sanctuary, for example, was clearly intended primarily as a parting gift for fans of the manga which concluded just as the first OAV was released. The Angel Sanctuary anime was never intended to be an introduction to a long series for fans just discovering the title.

Finally, sometimes anime episodes are simply a victim of circumstances. The Hakkenden OAV series, for example, is actually relatively long considering that most OAV series have no more than 6 episodes. However, simply watching it and seeing the dramatic changes in art design reveal the series' troubled production history. The Giant Robo OAV series is perhaps the best existing example of a troubled production, though. It took six years to release 7 Giant Robo OAV episodes in Japan! Furthermore, the same staff that created Giant Robo originally intended Makai Tenshou (known in America as "Ninja Resurrection") to be a theatrical motion picture, but had to scale back the title to an OAV release then apparently cancel production entirely after the second episode.

In simple terms, it's easy to gauge an anime series' popularity in Japan based on how much of the animation exists. Relatively few theatrical anime films get produced each year, so for a television or OAV series to get a theatrical movie, there must be massive popular support of the show in Japan. Series like Tenchi Muyo and Love Hina, with multiple OAV and TV series, only have as many episodes as they do because Japanese fans continue to watch and buy them. Series like Pokemon that have over 200 episodes continue to have more episodes produced and aired on Japanese television because there's still a great demand for them. However, series that seem popular to American fans, including Trigun and Gundam Wing and were either not popular enough in Japan to get second seasons, or ran their course and naturally died of waning interest in Japan years before they became massive hits in America.