What is so bad about being an "Am K'shei Oref"? Why does it negate the merit of inheriting Eretz Yisrael?
Seforno: Because a person who is stiff-necked sees only what he wants to see, and is incapable of turning his head to see what others show him, even when wise men show him that he is heading towards his own destruction. Consequently, his decisions cannot possibly be just and correct. 1
Seforno (on Pasuk 8): Moreover, his stubbornness prevents him from doing Teshuvah and the only solution is destruction - like we find after the sin of the Golden Calf. See Sh'mos 32:9, and below, Pesukim 13 & 14.