It is because that is what they do. They are a floury potato and quickly go mushy if boiled for long. Try a waxy potato such as Desiree, suggested by Snoopdog.

Yes as Bulba says, they are floury potatoes and inclined to go mushy. They are fabulous potatoes though; the best for roasties, mash or chips. We never buy anything else although they are pricey, so growing them will save a lot of pennies.
Desiree just don't hit the spot for us.

I steam all my vegetables, except sliced Runner Beans.
Boiling water in the bottom of the plastic steamer, lid on tight 9 minutes on High in the Microwave, couldnt wish for better. Once a week nip down to the Chippie at
11.45 a.m. He cooks me a whole Cod in batter £3.50.
Says "Do you want anything else with it ? -"
I say "No thank you I have homegrown nutritious steamed vegetables ready to complete a lovely meal."
Only the fish is too much for me. Cut it in half at home. Have the other half for supper.

Yes Snoopy. Until Christmas I brush the potato skins
in clear water. Also have a pair of potato gloves that
I rub the skins with sometimes. Found them in a cook shop
but they need hanging out to dry every time.
These are Pink Fir Apple Potatoes, lovely flavour.
Any variety. Use all the small ones too.
Just cut into approximately one inch cubes. Cook 9 mins
on high. Any left overs make a nice supper sprinkled with cheese, or put into a sandwich.
Happy eating !