Complicated answer, obviously but leading reasons:
1. Robinson played in a poor era for his skillset. If he plays during Showtime or in today's game, he's unstoppable. Unfortunately for him, he played in a slow era that allowed tons of contact. Bad luck.
2. His lack of low-post game hurt. But, really, it wasn't his fault. Robinson was 7-foot-1 with a 30-inch waste, he didn't have the body type to be great posting up. I'm not the world's biggest KG fan but I think it's silly when he gets criticized for not ever developing a low-post game. Players who are top heavy and skinny physically can't be great on the block. In the playoffs, having a big with a low-post game was deadly -- especially in the illegal defense era. Today, it's not as deadly due to the zone rules.
3. Bad roster composition. If you have a dominant big, you surround him with three-point shooters to create space for him. The Rockets had the best three-point shooting teams in the NBA in 1994 and 1995. , even the Magic figured out that surrounding D-Ho with three-point shooters would maximize his skills. Robinson was stuck on a team that couldn't shoot so the opposition could double or triple him without care.
4. Coaching was obviously bad. Teammates were pretty damn bad prior to Duncan. Sean Elliott is one of my favorite players ever but he was massively overrated. A two-time All-Star who never had a PER above 16.3? WTF, that doesn't even make sense in retrospect.

Leonard as a rookie had a higher PER than Elliott ever had. Robinson's best teammate during his prime was Rodman ... and he wasn't even interested in playing basketball.
I see merit in claims that Robinson didn't have a killer instinct or whatever. When you play in the Michael Jordan era and are compared to MJ's compe iveness that bordered on mental illness, Robinson was too self-aware to live up to that standard. But still, if he's in today's NBA, he's probably regarded as an above average compe or.
The redeeming thing for a Robinson fan such as myself is that as advanced stats become better and better, Robinson's legend continues to grow. From RAPM to APM to WS48 to even PER, Robinson grades out as one of the very best regular season players ever ... like in the top five. And if we knew in 1999 what we know now in terms of what wins basketball games, Duncan and Robinson would have been given equal billing. Robinson had the higher PER and WS48 in the regular season and tied Duncan in WS48 in the postseason. Robinson wasn't still in his prime in 1999 but he was probably still a top 5-7 player.

/classyrobinsonfanrant
