For loops are controlled by the value of N and the sizes of the all the arrays at the auto-indexing input tunnels. The smallest value controls the number of iterations. Your N is 10. But your array is a 1x X size array. 1 is smaller than 10 so the loop iterates 1 time.
I know it is one because you have a X sized 1-D array, so X columns. Then you use Build Array to turn that into a 2-D array. Now is has 1 row by X columns. The loop auto-indexes on the rows, so it only executes once.
Run your VI with highlight execution turned on and watch the data flow on the block diagram.
That's because you told it to run exactly once! Do you know how a For Loop works with one or more Indexing Inputs? It runs until the first of those present finishes. In particular, if you bring anything into a For Loop preceding it with a simple "Build Array" (that makes, say, a 1D array into a 2D array with one element), the For Loop will run once, say "OK, I'm done, I've processed the entire Indexed Input", and exit.
