Picard grew up throughout the series unlike Kirk.
It is canon that Jean-Luc Picard was a bit of a loose cannon as a kid. But he grew up and became a responsible adult who led the Starship Enterprise. You can say the same about Kirk except for the bit where he grew into a responsible adult.
He doesn’t really grow too much in the original series. (Again, if we were talking about Kirk in the J.J Abrams run of films, completely different story. He grows more in that first movie than Shatner’s Kirk did through his entire run.)
And maybe that was a failure on Shatner’s part as an actor or maybe the writing was there for Kirk to really change. But all we know is that Picard grew as the show went on. Kirk did not. And because of that, Kirk is a lesser captain than Picard is. Because who wants to watch a guy not change at all? I know that I certainly don’t. But watching the shifts in Picard was absolutely incredible.
It also helps that Picard had a much longer run to change. (But that’s not really an excuse since, again, Pine’s Kirk grows exponentially within that first movie and it’s about 2 hours long.)