The pattern here doesn't seem that unusual for the cast of a successful television show not to ever get another high-profile role; there are far more TV actors than TV roles to go around. In fact, many of the people on your list followed a typical pattern for television stars, which is to get one big break-out role, followed by scattered guest appearances in TV and movies.
Just a quick glance at IMDB shows regular television appearances by most of the people on your list, including Brent Spiner, Jolene Blalock, Michael Dorn, Alexander Siddig, Robert Picardo, and probably others.
Beyond that, typecasting probably does play a big part in why many of these actors find it hard to get new leading or starring roles. This was probably a bigger issue for the original cast, and possibly the Next Generation cast; by the time of Enterprise I think the "stigma" of being in a science fiction show was much less of a problem. The classic example of this is Leonard Nimoy, whose book I Am Not Spock was all about his typecasting, but it's easy enough to find most of the TOS and TNG actors complaining about being typecast at some point.
The fact that the early casts kept doing movies, and thus kept going back to their characters, probably didn't help much. On the other hand, the movies, and convention appearance fees, likely kept them making enough money that they weren't all that bothered by the lack of other work. James Doohan explains how he came to accept his role as Scotty, and even learned to make a living from it for the rest of his life:
In 1973, he complained to his dentist, who advised him: "Jimmy, you're going to be Scotty long after you're dead. If I were you, I'd go with the flow."
"I took his advice," said Mr. Doohan, "and since then everything's been just lovely." src
Many of the original series cast were already well into their acting careers anyway, and just decided to retire. DeForest Kelley, for example, had been acting for almost 20 years before he was cast on Star Trek. Ironically, he was actually being typecast as a villain, and chose to play Bones to "break away" from that. However, he was never bitter about being re-typecast due to Trek:
People have asked me, you know, about Star Trek and how I feel about it having typecast all of us [...] I feel so fortunate to have had the experience that I had in the motion picture business, that Star Trek just became cream on top of the coffee for me. src
Others would eventually come to grips with their typecasting, and take advantage of their status as science-fiction icons to reappear on later genre shows, including: Fringe (Nimoy) or Heroes (Takei, Nicols) or Stargate (Blalock, among many others). As Nimoy explains, this was mostly just learning to accept what they had:
I was so heavily typecast and so heavily identified with the Spock character, it was difficult to draw attention to the other work that I wanted to do. But once we started to make Star Trek films that tension went away. I became very comfortable with working occasionally on a Star Trek movie and doing other work as well. src
Also, a lot of these actors simply chose to do something different with their careers after leaving Star Trek. Of particular note LeVar Burton and Roxann Dawson went into directing, and Jonathan Frakes began producing shows of his own, all of them relatively successful at it. Many of the actors, people like Patrick Stewart and Avery Brooks, were active in theater before their roles, and went back there afterwards. (Avery Brooks, IIRC, has also released an album.)
Overall, I think the impression that Star Trek actors have any more problems getting work than any other actors is probably just cognitive bias: you are hoping to see those actors again because you like them, and when they don't show up, you wonder why. But in reality, I don't think it's any worse than anyone else.
