Perhaps one thing you may be forgetting is that, when the Brexit negotiations between the UK and the EU started, the Conservative Party had a healthy majority in the house of commons. Therefore, there was some level of confidence in the UK Government that, as long as they could negotiate a deal with the EU that was acceptable to the Tory Party, they would be able to use their majority to get it through Parliament.
However, in 2017, Theresa May made the (in hindsight, unwise) decision to call a general election. At the time, she was confident that it would boost the Tory Party's majority; however, the result was the exact opposite - the Tory party lost seats and lost their majority (even though no other party gained a majority either, i.e. it was a hung parliament).
As a result of that general election/hung parliament, the balance of power in the UK Parliament shifted. Now the Tory Party lacks a majority and requires the support of the Northern Irish DUP in order to get any legislation through. So, the outcome of this disastrous (from the Tory point-of-view) election has given Parliament considerably more power over Brexit than they had at the time the negotiations began.
In summary: the political situation has changed.