it says in the Polearm Master - Feat: 

• When you take the Attack action and attack with only a glaive, halberd, quarterstaff, or spear, you can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary attack. The weapon’s damage die for this attack is a d4, and it deals bludgeoning damage.
• While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach.

Was wondering why the weapon pike, is not listed in the first statement, But is in the last one. Maybe just forgot to put it in?

Thanks in advance.

Because a pike is too long for the first part to work. The shortest ones were 10 feet long, and they could get up to 25 feet in length.