The bottom line is that Paradox needs to make money. How they do it, it is up to them. While games like WoW can be wildly popular while strictly multiplayer, a study is often cited that about 90% players prefer singleplayer, or don't even touch multi. Now, this data might be outdated by now and it's hard to track down to actually show anyone, so it's hardly a good argument.
So it boils down to the customer base. Every company must make it's games as accessible to the wider audience as possible. Paradox has chosen a niche market, and while the customer base is smaller, they benefit from no serious competition - they are the best in this biz.
So how can they pick up a casual gamer? A casual gamer should be allowed to sample the game in the time he has available. Multiplayer in Paradox games is a fairly time-consuming affair, isn't it? Furthermore, it helps to know the players beforehand, as you will be spending quite a lot of time with them. A casual player, as the name implies, probably won't invest as much interest in the game until it really sucks him in. And that's one of the roles of SP.
Therefore, neglecting singleplayer to create a better MP experience is (if my theory is correct

) counterproductive - while those that do buy the game will have a richer online experience, they may find the player base smaller, because fewer people decided to try multi because of... the poor SP experience.
This is pure speculation, of course, so feel free to correct me/disagree.