How isn’t parliament still sovereign?

While, in practice, devolution has indeed acted as a constraint on Parliament’s political sovereignty, it is important to remember that, legally, Parliament retains sovereignty.
While the Scotland Act (2016) formally recognises the existence of the Sewel Convention, this does not make it legally enforceable. In January 2017, the devolved administrations argued before the Supreme Court that the Sewel Convention required legislative consent motions to be passed before Parliament could give the UK government power to trigger Article 50 of the Treaty on European Union and begin the process of withdrawing from the EU.
However, the Supreme Court ruled that, as important as the Sewel Convention is, and as substantial as the impact of Article 50 might be on the devolved institutions, it had no power to enforce constitutional conventions. While the UK may function as a ‘quasi-federal’ state, it is legally very different to federal states, where the constitution recognises that sovereignty is shared between federal and regional governments.

Unlike senior courts in countries with codified constitutions, the UK Supreme Court cannot strike down Acts of Parliament.
While ‘declarations of incompatibility’ can place Parliament under considerable pressure, it is still up to Parliament to decide how to respond.
Hirst v the United Kingdom (2005), the ECtHR has repeatedly ruled that the UK’s blanket ban on prisoners voting is a breach of their human rights.
Yet, despite repeated rulings, Parliament has refused to amend the law.
In February 2011, the House of Commons even chose to pass a motion against prisoners gaining the vote by 234 to 22.
While it would be politically difficult, the UK could withdraw from the ECHR, and, while the HRA may function similarly to the entrenched US Bill of Rights, it can be expressly repealed just like any other law.

A Prime Minister with a large majority in the House of Commons may have remarkable political power, but they are not a monarch, with absolute legal power, who can simply declare new laws into existence.
Following the 2016 EU referendum, the Conservative Government was confident that it could rely upon royal prerogative powers to trigger Article 50 of the Treaty of European Union and begin exit negotiations.
However, the High Court and Supreme Court both ruled that beginning the process of withdrawing from the EU would ultimately have the effect of rendering the European Communities Act (1972) meaningless.
As only our sovereign Parliament can amend or repeal an existing Act of Parliament, new legislation was needed, regardless of the referendum result.
Similarly, even a Prime Minister with a large majority can never be completely certain that their proposals will make it through Parliament. Political power can be fickle, and the executive can never know when our sovereign Parliament may put a break on their plans.