Their teams didn't have the players Ten Hag has. When De Boer took over at Ajax they were in the midst of a financial and organisational crisis, and were making cuts, had a salary cap of 1 million euros etc. It's a miracle he managed to win the Dutch league 4 times in a row. The team balance was completely off, too reliant on youth alone. The average age was like 21 years old at the end of his reign, the team was hollowed out. After that Ajax slowly became financially more healthy, it's only been the last 3 years that Ajax has started spending more again and signing quality and experienced players like Ziyech, Neres, Tagliafico, Blind and Tadic. Tadic is reported to get at least 3 million euros a year, which was impossible in the years before that. Bosz also left before being able to fully benefit from that, although the first signs of a resurgence were there, the average age that European final was just 22 years old still, it's 24 now.
Don't get me wrong, I really rate Ten Hag as a manager. But the extraordinary season Ajax has had isn't just down to him, it's a result of everything falling into place, including the manager. 7 years of Overmars and Van der Sar getting the club in order, paying off debt, saving money. Reorganising the club aswell, appointing the right people, like new head of scouting Veldmate, he's doing a great job scouting South American players. And the last 2-3 seasons they felt comfortable spending more again, and pushing for results and prizes.