Because, fresh pasta has twice as many carbohydrates as dry pasta.
In the Slimming World rules, you will see, you can only choose dry pasta to have as a free food. When you cook fresh pasta, it stays the same size and the amount of carbohydrates in 100g of fresh pasta is the same as the amount of carbs in 100g of dry pasta. But when you cook dry pasta, it doubles in size, therefore having half the carbs on your plate. And as Slimming World is based on Low Glycemic and low fat foods, this would stop you eating too many carbohydrates in a single portion.