Originally Posted by
Scots_Al
But for a better explanation:
https://www.bbc.co.uk/news/uk-41059610.amp
Tax is lower on 3.5% and below drinks. The ABV on a label is allowed to be out by 0.5%, so your 4% labelled beers are actually being brewed at 3.5% to lower the tax burden.
There's also genuinely an increasing market for lower strength beers too.
That last point is certainly true in the real ale space, where better tasting session ales are very desirable. I had a glass of 13.5% stout as my cheeky shopping tipple, today. For some reason, the barman would serve me no more than a third of a pint.