I think it comes down to supply and demand.
Generally about five second tier players at each club make an appearance in the NRL each year. This year the Roosters didn't use any, while Parramatta used six (with permission) and the Tigers used nine (also with permission). Most other clubs used five.
So depending on how much flexibility clubs have within their top 25, and specifically within the backups in the top 25, clubs will sign second tier players to round out the flexibility of their squad. There's only so many second tier players you can sign at each position before you're just wasting money. Likewise for the players, if they have any ability at all (even for second tier players) then they don't want to be stuck in a 30 man second tier squad, hoping to be one of roughly five blokes who get a chance in first grade that year.
So if you're a front rower (for example), and you haven't picked up a top 25 contract anywhere, then you don't want to sign a second tier deal with any club that already has a better front rower than you in their second tier. You will go somewhere where there's more opportunity to play first grade.
Unless you're not that interested in playing first grade and just want to drag your arse around the NSW Cup for $70k a year.