They may simply not be interested, may not care for a romantic subplot in general, or may be waiting for their preferred squadmate romance in the sequels like Miranda, Tali, or Garrus.
There are many reasons Shepard may not want to begin a romance with one of these characters. Related: Mass Effect 2: How to Romance Miranda Lawson None of these squadmates can join Shepard on their mission in Mass Effect 2, but if Shepard begins a romance with one of them, it can create awkward situations later, especially if Shepard wants to move on. Only three squadmates are romanceable in Mass Effect: Kaidan Alenko for a female Shepard, Ashley Williams for a male Shepard, and Liara T'Soni for both male and female Shepard.
Based on these kinder dialogue choices, Shepard can inadvertently begin flirting and even start a relationship with a squadmate without realizing it. It can be remarkably easy for squadmates in Mass Effect to get the wrong idea, just because Shepard chooses Paragon dialogue options.
