Well, here's my take on the in-game situation as well as why Bioware wrote the game this way:
In-game, if Shepard romances Ashley or Kaidan, they didn't really have more than a few months together before a two year absence, where Shepard is presumed dead. As far as Kaidan is concerned, he fell in love with his commanding officer, went through hell with her, saved the galaxy, etc. If the Cerberus missions were completed, Shepard and Kaidan both agree the organization is misguided at best and at worst a terrorist hate group. After seeing his love for the first time in two years, Kaidan is shocked, and has to figure out if the two of them should be an 'us' again. Unfortunately, the added complication of Cerberus in the picture is enough to put a huge damper on the relationship. Both Kaidan and Ashley are passionate,and both would react that way to Shepard, regardless of their relationship, romantic, friendly or strained. In story, it does make sense for either of them to have moved on in the Alliance and even possibly see other people (eh, this part makes me mad, but I can begrudgingly understand why well-meaning friends of Kaidan's would want him to move on).
I'll let others speak for Ashley, since I usually don't spare her, and for Liara since I haven't played that import yet.
On the other hand, Bioware is a company that wants to market to as many people as possible. Writers can develop a story in such a way that can explain why certain events happen, or certain characters do things. They reduced the role for story reasons, but also to improve character romances and interactions. Based on the fan feedback, they may or may not bring characters back. If not enough people care about Ashley and Kaidan, I'm afraid that we'll see a minimal role at best, and possibly even nothing at worst. Liara is clearly a favorite of both fans and the writing team, so luckily she's going nowhere.
Most likely, they have something up their sleeves fro Me3, and we'll just have to wait. I'm most interested to see how they will incorporate all the li's returning, from both games.
While I'm extremely disappointed that my favorite two squadmates are off turning into their mother or too busy with Alliance stuff, I really do enjoy the game, fully expecting them to make a full return in Me3. That's one thing that keeps me going, actually, a nice, happy resolution
I'm really hoping for this, esp. since when I found out the li's were not returning in Me2, the character of Thane got me excited for ME2 as a romance option, but now that he most likely will not be in the sequel, I'm back at square one.
This may seem petty to some people, but, tbh, what drew me to Bioware games was the whole romance, amazing adventure saving the world with your love at your side thing. I've always had happy endings, unless playing an evil character, but that's just to see other aspects of the game. Anyway, if Me3 moves even further from this, well, I'll be one of many extremely pissed off fans.