Because they do not have the same level of difficulty.
1/2 limit is the 2nd smallest level on Party but 1/2 no limit is the 4rd smallest level. A lot of B&M poker rooms have 3/6 as the smallest limit game but 1/2 as the smallest no limit game.
In addition, you need a larger bankroll for NL. The standard for limit is 300BBs but for NL it is 2500BBs (25 buyins).
Just to clear the confusion ; it's 300 big bets for limit and 2500 big blinds for NL (since there are no big bets in NL)