Why do you think experienced senior HR professionals from the public sector often struggle to secure interviews for private sector HR leadership roles?

I was a part of a private company which took over a council owned/operated business many years ago, I'd suggest there are a number of reason.

1.  Little if any commercial acumen.   Staff being paid extra hours, 'guaranteed' O/T .   Staff sharing out O/T when it would have been cheaper to bring in a P/T employee.  etc.,   "Well we didn't want to rock the boat" , sort of attitude.

2.  Didn't want to do anything which might upset the Union.   "Well we've never really bothered to try to change XXX   as the union wouldnt, or might not  like it."

3.  Archaic practices.  "Well we've always done it this way".   

4.  Training which was never properly reviewed or assessed - or no longer needed, or/and out of date. 

This of course is my own opinion and based on my experience only.

I echo David's feelings. And, for what it's worth, I think the same applies in reverse - I think it is difficult for HR in private to get a foot in the door of public sector.

HR in the private sector:

• More ability to effect change - you have direct access to the powers that be and a decision can be made in one Board meeting and enacted immediately after.
• Higher risk appetite - you can make commercial decisions about whether to follow due process, short cut through settlement, or just short cut and accept the risk of a poor outcome. Usually the reputational/public opinion risk is low because most of the time, there is no press coverage
• Less pressure to redeploy/make "reasonable" adjustments - private businesses are often (though not always) smaller or more streamlined, so "reasonable" has a much narrower definition. While the law applies equally, the perception of available resources is lower.
• More control over role definition - what is expected of managers can be set by each business or region or even each team. You can vary this depending on capacity and capability. You can put interventions in place to build capacity and capability relatively smoothly.
• More focus on commerciality - HR must demonstrate that interventions (and indeed themselves) represent return on investment. And a lot of the work is coaching managers on commercial awareness and achieving commercial gain through people solutions

HR in the public sector:

• More structured and process-driven - more internal precedents to consider, frameworks to rely on, need to work holistically within procedures
• Lower risk appetite - driven by the need to protect reputation, increased public scrutiny, inability to throw money at the problem to make it go away, and (arguably!) a higher focus on operating ethically
• Higher expectation of what is "reasonable" - with so many departments, unless a role is specialist, there is often a redeployment opportunity somewhere.
• Roles are clearly defined and hard to change - again, set procedures and policies, set expectations. Any deviation can create wide-reaching risk as often there are many, many people in the same sort of role and seemingly innocuous actions could lead to claims for moving up a pay band or accidentally create a hole elsewhere that isn't noticed until someone leaves etc.
• Less exposure to commerciality - budgets are set at such a high level. Many managers are "commercially aware" because they look after their own budget - but that's not the same as being in a Board meeting or cross-department meetings regularly. They don't see the whole business until they are already very, very senior. And they are less likely to have to justify the need for an HR department...

There are loads of positives that come with employment in both and loads of challenges. But they are different challenges and I think to be able to make the move, you have to demonstrate you know what those challenges are and how you would overcome them.

I think it is the experienced person's version of "must have experience for this entry-level role" - can't get experience unless someone gives you a role, can't demonstrate actual real lived experience of the public sector without being in the public sector.

It all comes back to supply and demand. If you have two applicants for a public sector role and one has experience in the public sector, you will pick that one - unless you are deliberately looking for change and up for taking a risk on whether the private sector professional can adapt quickly. See "lower risk appetite" for how likely this is