The costs are greater with Spreadbetting - so you need a larger movement (in the right direction!) to make a profit. For very small profits ( such a day trading where presuemably you are looking for a lot of small profits) a CFD is better as the costs wont eat in your p&l as much, and of course what could be a profit under a lower cost CFD could be a loss in a spreadbet- in which case any tax due would be welcomed!!
Yes, probably because the Spread Betting company, who must pay taxes on profits, is passing on the cost to it's clients. But my answer is more general. The reason is that Spread Betting is non-professional and aimed at the public betting pennies or traders with mediocre aspirations. Professionals, I mean REAL professionals as opposed to full time traders, don't spread bet.