Heian Nidan was originally taught as the first Kata for new students. Its connection with Kanku Dai  (the Central Core of Shotokan Kata) is quite obvious, as the first half of Heian Nidan is almost exactly the same as the first half of Kanku Dai. 

The Kata uses a large selection of new techniques:-

Double handed blocks
Sword hand blocks and strikes
Spear hand strike
Back fist strike
Two different kicks
Reverse punch
Reverse side blocks


This number of new techniques makes the Kata more difficult to learn than Heian Shodan and may be the reason why Funakoshi changed the order of the two Kata.