Because Sissoko was out of contract in 6 months and therefore the bidding war was based around his wage, and not his valuation/transfer fee. I imagine that agents bargain for the fact that a club should be willing to pay a higher wage on a player, if they didn't have to spend any money on a transfer fee (or a pittance to bring him on 6 months early after the contract is already signed, a la Lewis Holtby and Spurs). That's not in play with Belhanda, because he has not only a full year, but 2 years left on his current deal. That removes a lot of leverage when it comes to bargaining for the highest possible wage.
Don't get me wrong, IMO we're not signing him, but I don't think that Belhanda wrecking the wage structure is the issue.