Why is the mast connected to the ski and not the deck?

There are really three reasons for the placement of the mast on the ski rather than the deck.

The original ski was much like the mono ski's we saw in Saskatoon, very narrow, very tipsy, but they do go straight real fast, without a skeg and one foot in front of the mast base. tried it with my first prototype  but I didn't like the position I had to assume to sail that way.

Time and testing continued and showed the ski to be much more stable in deeper snow and when trying to jibe with the mast lower, that's how it evolved.

So the first reason is load vectors: with the mast on the deck, it's forces are directed downward, forward and a lot to leeward,  HOWEVER,  the center of axial rotation (the center line of the ski) and center of resistance is some 3 inches below the mast base and your feet.  You now must compensate for these forces with your feet.

Placing the mast on ski results in greater form stability (universal close to axial rotation) and increased roll response (the 3 inch lever of deck needs only direct the ski and not have to compensate for asymmetrical mast loads), both desirable traits when sliding and jibing.

The second and third reasons are rather interrelated; structure and flex; There must be a structure to support the mast base, and if that structure is attached to the ski it can eliminate flex in the ski, and skis turn as a result of the combination of side cut and flex which create the turning radius of the ski.  That's the reason the new Carve Windski now uses a large amount of side cut and a single riser with a minimum hard point on the ski to promote flex and a tighter turning radius.   If as has been suggested the mast is supported by a cantilevered prow the ski flex will still be limited to the height of the supporting structure and deck. There are also fabrication complexity and costs.

There is one additional reason I haul out with an extra glass of wine, "... it allows better closing of the "gap" between the foot of the sail and the ground.  Do you know how much power is lost to the low pressure side of the sail in that gap?  ...Well yes, I'll have another glass of Cabernet, thank you!"

As a side note, reasons 2 & 3 also apply to the individual mast base locations  on the Windski as apposed to a track.  Something lots of windsurfers ask about.