More tBTC used for financial purpose which makes less place/people have ability or liquidity to continue offer tBTC for free. If more project ask for tBTC, i expect it'll also affect raise difficulty/hashrate on tBTC network.
I believe that sometimes teams want to do testing as soon as possible, so they would rather pay tBTC than spend time to find for free.
Is it possible that one of the shortcomings of testnet coins comes from the fact that hardly anyone returns them to the network after testing? They just leave them where they are because they are worthless.