Why do Bhikkhunis have so many extra Vinaya rules? The Vinaya contains 227 rules for male monks and 311 rules for female monks. Is it known why so many additional rules were put in place for Bhikkhunis?
Thanissaro Bhikkhu, in Access to Insight, says,


  Thus the Bhikkhunī Pāṭimokkha contains 85 rules for which there are no
  direct correspondences in the rules for the bhikkhus. Some writers
  have interpreted these added rules as sign of an attempt to oppress
  the bhikkhunīs unfairly, but it should be noted that:
  
  
  more than one third of these extra rules were formulated to protect bhikkhunīs from being the direct recipients of the abusive or careless
  behavior of other bhikkhunīs;
  two of the extra rules (Pācittiyas 6 and 44) prevent bhikkhunīs from putting themselves in a position of servitude to bhikkhus or lay
  people;
  according to the rules' origin stories, all but three of the extra rules (Pācittiyas 59, 94, and 95) were formulated only after
  bhikkhunīs complained to the bhikkhus about an errant bhikkhunī's
  behavior.
  
  
  Tellingly, these last three exceptions were formulated after
  complaints initiated by the bhikkhus, and they touch directly on the
  formal subordination of the Bhikkhunī Community to the Bhikkhu
  Community. However, they are counterbalanced by two rules exclusive to
  the Bhikkhu Pāṭimokkha — NP 4 &amp; 17 — that were formulated at the
  request of bhikkhunīs to prevent bhikkhus from abusing their position
  in the hierarchy in a way that would interfere with the bhikkhunīs'
  practice of the Dhamma. For a more detailed discussion of the checks
  and balances in the relationships between the two Communities, see The
  Buddhist Monastic Code, volume II, chapter 23.




For what it's worth there is a further set of opinions, published here: Extra rules. This too says that the rules "primarily dealt with the protection of the nuns".