Votes come from other team coaches and captains. It's very possible that they don't rate him particularly highly for whatever reason.
Also, as I said in the Wortho thread:
Each All-NBL team submitted by the voters must feature three outside players and two inside players.
Positions come into play a little.
The other factor is that MVP voting is done during the season, after each game. I think the All-NBL team voting is done at the end of the season.
Here are the bigs named ahead of Simpson. Try to imagine the thinking process of coaches and captains:
Khazzouh - NBA trial, considered one of the best in the league, fantasy stud
Wortho - Boomer, big salary, winning history, scorer and defender
Tragardh - has his flaws, but was spoken of as an MVP candidate during the season, quality scorer, dropped away as Tigers imploded
Wilkinson - champ last year, pole position for champ this year, works hard, has definite range
Johnson - likely MVP of his team, top five in scoring, ranked in blocks and boards
Wagstaff - solid contributor for an excellent team, versatile and reliable, top five 3P%
IMO, the only two Simpson has a chance against are Tragardh and Wagstaff. Wagstaff they know is a winner and that might weigh on their mind.
Another one who would've been close was Jerai Grant whose numbers are not miles off Simpson's. Also, perhaps Holmes?