If you are Thomas and have just added more money to the pot, would you fold? We can't see his face though but I can just imagine how disappointing it would be to learn Steve just have KK
I would never do that if I were on his place. What a stupid move he has taken, he could easily win that money without any problem but he Steve got him with his bluffing skills. In poker if you don't understand your opponent's bluffing style then you'll most probably end up losing money.
Thomas got a fixed straight but he missed it because Steve's bluffing style got him and might got scared by thinking that what if Steve holds Queen and King card. In any gambling one should understand that strategy won't work without luck, in this case Thomas was lucky but his strategy was wrong and that's why he lost the hand.