James Olsen is being re-imagined in this way because of the licensing issues over Superman and the storytelling requirements of a Supergirl adaption - Superman's connection to Kara is a very important part of her character, but they don't have access to the character - so as far as this show goes, James Olsen = Superman. That's why he's projecting the strong, confident and caring mentor personality, because he's Clark's stand-in for the show - for example, take the last scene from the trailer where he gives her the Kryptonian baby blanket - make a couple of tweaks to the dialog and switch him out for Christopher Reeve, and the scene works exactly the same.
They couldn't really have Kara only communicating with Clark with text messages, and it wouldn't really fit the style of the show with all it's call-backs and references to the comics to just invent a new character who was a close friend of Superman who'd act as his stand-in and go-between with Kara.
The traditionally geeky Olsen personality wouldn't work in this show because of the role he has to play, plus it'd overlap with Kara to a certain extent, as well as her tech friend Winslow Schott.