Why would an employer not be interested in salary sacrifice?

'Sacrificing' gives you a lower contractual rate of pay in exchange for some fixed contractual benefit. So you can see why en employer might not want to provide that benefit if people don't actually show up to work and their normal protection from such abuse would be to have the person's pay docked.

But generally although it sounds like a win-win for employee and employer, they can be costly/burdensome to administer, especially for smaller firms. It's not just doing a bit of paperwork for a couple of employees and diverting cash, but a whole exercise to inform and educate the workforce on how it works (to avoid potential employees later claiming they didn't understand it), administration of potentially running multiple schemes / plans (e.g. auto enrollment non-sal sac and the sal sac one), making sure people do not end up under the minimum wage and so on.

Depending on how it's implemented and the figures involved there can be negatives for employee too, e.g. lower headline salary or wages when you are applying for a mortgage, potentially lower contribution-related state benefits if your NI contributions drop too low to qualify and so on. And for higher rate taxpayer the employee NI saving is only 2% so there's less incentive for staff to want to do it and therefore lower potential savings on the table for employers.

If it's just you wanting to put away £100pm, that's only £13pm for them in saved NI and that typically does not pay for the advice on setting up and administering a company-wide scheme and the extra admin and compliance involved for them on an ongoing basis. £13 a month of extra revenue or saved costs is not a goal that most firms would find it's worth the business disruption to achieve. Obviously all the £13s add up so at some level it becomes worth doing but it is a fact of life that some small firms find running flexible benefits packages a chore.