Money is one factor, and I think Ward Prowse would cost quite a bit more. We know money is going to be tight this window.
Fitting into our team and our style is another. Currently we pretty much only have Sissoko that adds a physical aspect to central midfield. If a new signing is to thrive in our system next to Lo Celso, Winks or Ndombele I think we need that physical presence. I don't think Ward Prowse would be a good fit in that regard.
Finally I think defensive stats are rather hard to interpret. The Athletic presented stats compiled by smarter scout, adjusted for his position. Scores out of 100:
Defensive:
Tackling ability 90
Ball recoveries and interceptions 97
Disrupting opp. moves 84
Those aren't stats that are available to us so I don't know how Ward Prowse would have scored. Good numbers though.
As an aside his worst was the possession state ball retention (10) and his possession stats were in general rather poor. Interestingly his attacking stats (xG from shot creation at 75, xG from ball progression at 77 and reception in opp. box at 79) were all rather good. Perhaps indicating a more direct, less possession oriented style.
The story was about him being linked to Everton so they compared him to Tom Davies, Delph and Andre Gomes. Was better than all of them in attacking stats (as well as defensive, by some margin looking at the whole).