Several issues:
First -
Per the data sheet, page 18:
To enable programming of the RDAC register, the write protect bit (Bit C1), of the control register must first be programmed by loading the serial data input register with Command 7
per the data sheet page 20 table 14, shows that by default C1 is 0,
0 = wiper position frozen to value in 50-TP memory (default) , or midscale if not previously programed.
See table 12's example sequences to get things moving, by initially sending 0x1C03. I believe you could substitute with 0x1C02, if you do not want to enable programming and just want to update on the fly.
Second -
Also looks like from the data sheet the Din is sampled on the falling edge of the clock. you need the following in the setup
SPI.setDataMode(SPI_MODE1);
Where often this may appear to work in MODE0(Default) with slower data rates.