Those numbers are pretty much in line with how many people complete single-player games in other genres, actually. An Activision executive
estimated to CNN in 2015 that 90% of gamers don't finish games, the now-defunct game tracking website Raptr reported that
only 10% of players completed the final main story mission Red Dead Redemption, and a GDC talk by two industry veterans
detailed low game completion percentages across many great games. The Guardian
also reported very low game completion numbers.
If half of all players who touched the mode got through the first two acts, Tiburon actually did pretty well, and they are about on-par with how the rest of the industry is doing with respect to how well they engage players in story-driven single player modes.