I don’t think that it’s just down to inexperience.
I think that only two of the New Labour ministers had ever served in government before 1997 (Jack Cunningham and Michael Meacher), but most of them had come up through activist and/or academic backgrounds during the 70s, before moving into parliament, and being of age in 1997 to be in the cabinet or government.
At the moment though, there’s a huge dearth of talent in the Labour party in the Gen X cohort. The NOLS/Councillor/PPC/MP fasttrack that was established and tightly controlled under New Labour has led to a clutch of lacklustre and careerist people in parliament - the kind of people who wore suits and took briefcases to university lectures rather than campaigning or embedding themselves in community action. Perhaps they would have become better had they been mentored more, but most of the New Labour ministers disappeared from the Commons after the expenses scandal.
The group of millennial/Gen-Z MPs coming up behind (eg Whittome etc) have more potential in this regard, I think.