It's a very good question aog: I suspect the answer is partly because that is the way "they" operate, and quite possibly actually drawing attention to it is part of the aim.
When Litvinenko was killed, it's thought they messed it up and that if the thing had been done properly he'd have died a lot more quickly, apparently of natural causes - perhaps with more than a hint of suspicion, but with no one able to prove anything. After all, even with the way it happened it took weeks to cotton on the actual method used. With this, it's very strange: unless Mr Skripal was up to something (which he may well have been) he would not have been the threat people like Litvinenko, Perepilichny, Berezovsky etc were to the Kremlin. His case was "closed" as it were. On the other hand, he was a very easy target for anyone wishing to make a brash statement.
It's also possible that like Litvinenko this was also bungled, although I'm not sure whether it is possible to attack someone with a nerve agent such that it looks like natural causes. It seems unlikely.