Because, if a map is to be rotated meaningfully, the text needs to be on a separate layer, so the text remains upright as the underlying map is rotated.
Moreover, the text needs to be not just one layer but each piece of it (e.g. each airspace label) needs to be "attached" to a particular piece of airspace so that as the map is rotated the label can be redrawn in the right place.
And the CAA does not publish its VFR charts in a layered form. They let them out only as a single-layer flat graphic, and that is how Memory Map resells them.
Companies which produce rotatable aviation maps are Jeppesen (whose maps are used on most GPSs), PocketFMS, and I think 1 or 2 others. But I don't think any of the national-CAA-produced VFR charts are available in a layered form suitable for rotation.
There is a separate issue with rotating a graphic in that there is an inevitable loss of resolution - unless the rotation is 90/180/270 degrees in which case you can do it precisely. But I don't think that is the major issue here.