Because that would mean different prices for the same products on either side of the border, it requires an opt-out from the Internal Market Act.

Source: https://www.heraldscotland.com/politics/23381947.uk-government-set-refuse-drs-internal-market-opt/?ref=rss

The formal process for excluding the deposit return scheme regulations from the Internal Market Act is well underway.

Source: https://www.inverness-courier.co.uk/news/in-depth-greens-co-leader-addresses-incorrect-assumptions-303160/

The Internal Market Act allows for policy divergence if it is included in one of those common frameworks.

Source: https://www.heraldscotland.com/politics/23385388.alister-jack-doesnt-know-devolution-works-says-lorna-slater/?ref=rss