I put it down to a risk averse attitude....
Odds are (with conventional wisdom) that a younger, less experienced and less physically developed player will make more errors or be more easily dominated than an older, more experienced and street wise pro.
I'm sure I read somewhere that Bulut does have a bit of a track record of bringing youngsters through though, so maybe, just maybe we'll see a little bit more of it going forwards, especially if the team is looking relatively comfortable as far as league position goes?
It's definitely easier for a less experienced player to step into a team playing well and with confidence/a rhythm than one struggling, lacking in confidence and with every chance of team mates pointing fingers and the crowd getting on their backs quickly when passes are misplaced or when getting caught out of position.
Of course, it's a catch-22, if you don't play them, how can they improve but can you risk playing them if the stakes are so high??