A few reasons. I've been watching him since he was 17.
For starters, he has just never been that good. As a junior he was really good at running the ball, but never really had a great passing or kicking game. He just got lucky that he played next to Fifita for both school and club. His two moves were running the ball or "pass it to Dave and watch him run over 6 blokes". It was easy to look good playing next to Fifita against 17yo kids, and carrying that combination in to rep teams made it all the better for him.
He and Paix were the golden boys in the Broncos system, but Paix was always the better player. They even got special permission for Boyd to play Q Cup at 17 (and he was terrible). Seibold came to his senses and shipped him off to promote Dearden, then suddenly he is playing NRL for the Titans despite never putting 2 good games together in the Q Cup. Whenever the Titans sent him down to Burleigh he was diabolical. Last year I think they won almost every game Boyd didn't play in, but lost nearly every game he did.
He's basically the anti-Jamal Fogerty. One had years of dominating the Q Cup and they refused to give him an NRL opportunity. One had a short stint of ineptitude in the Q Cup and was promptly called up to the NRL.
The Broncos turning Paix into a hooker to make way for Dearden is also a bit of a travesty imo, but that's a different story.
Boyd did actually play a couple of good games in the NRL at the back end of last year, and these days he is an ok squad player. I just don't see him as the guy to push a team around the park for 25 weeks. It's a Kyle Flanagan situation. Over-hyped and given a million opportunities, but imo he didn't earn them (with form I mean. By all reports from people I know he works his ass off, so that is a positive).