Which factor is often cited as a reason for the difficulty of passing private members' bills?

The requirement for major cross-party support is often cited as a significant factor that contributes to the difficulty of passing private members' bills. Private members' bills are proposed by legislators who are not part of the government and typically struggle to gain attention and prioritization in a legislative environment dominated by government agendas. Without substantial backing from multiple parties, these bills can easily be marginalized.

Cross-party support is crucial because it helps to ensure that a bill has enough votes to progress through all the parliamentary stages. If a bill only garners support from a small number of legislators, it is unlikely to make it past initial readings or committee scrutiny. The more broad-based the support, the better the chances of successful passage; this includes not only securing votes but also facilitating collaboration and debate among differing political groups, which can enhance the credibility and viability of the proposed legislation.

The other options, while relevant in different contexts, do not encapsulate the primary challenge of garnering enough political agreement across party lines, which is essential for advancing private members' bills through the legislative process.