The roster is constructed differently this year and there's no regular slot for Keirsey to get at-bats. He's on the team because he has very good speed and would appear to be at least an above-average outfielder. He's 28 years old in a few days and very few 28-year old rookies become mainstays. Maybe he's an exception, but I'm very skeptical.
I've had the chance to watch more minor league baseball in the last few years and it is striking to me how different it is from the major league product. Balls aren't fielded that routinely get turned to outs even for the worst defensive teams in MLB. Mistake pitches seem common and few pitchers have the command to exploit weak spots that we see routinely with the major leaguers. I have close to zero faith that good numbers in AAA will translate to the majors. Keirsey put up very good numbers last year, but hasn't hit at all as a major leaguer Every year there are examples of this and maybe Mickey Gasper is another example.
Minor league field staff see guys and recommend who should be promoted. If they don't do a good job of evaluating talent, they don't keep their jobs. Despite pretty average numbers the Twins were quick to promote Keaschall, but not Keirsey or Yunior Severino, who put up good numbers in the previous two seasons. I imagine the Triple A field staff saw red flags for the latter two, but not for Keaschall.
Underdogs like Keirsey, Eeles and McCusker have to beat the door down harder and produce more than guys with pedigrees and they probably get fewer chances.