Because your sketches do not tel to the program an easy way ! :) So you cannot loft (blend) a 3d sketch with another sketch! You need two 2d sketches or of some entities on two plans.
One thing few people know... be sure that the two sketches have the same number of points. See picture and SW2015 part.
The solution often seems easier after she was found!
I think the drawing man make an error. The designer must know that a craftsman can make what the machine can not always do. Software like Solidworks does not know everything, but it can do everything that the machines do!
So I changed to a minimum the problem by adding an element without impact but provides a logical solution.
Attachmentsloft 5.SLDPRT, 141 KB
Can you please help me with this problem I have posted on grabcad ?https://grabcad.com/questions/how-to-use-boundary-surface-function-in-solidwork
know my drawing does not match the design that you stand to make exactly the same. I told you there is an error in the smoothed boss. When running the boss with two neutral element which does not have the same number of points, yes smoothing find a solution but this solution results in an ugly asymmetrical shape. Look closely at my attached file. The only thing I changed is that I added a very small curve to that smoothing perfect.