There are some reasons why it wasn't build with VLS in place from the start.
First of all they wanted a more compact hull which grew larger and larger with each iteration. At the start they didn't put much thinking about the weapon and there were some problems with answering the question of 'what should we put there?'. It seems like they decided that it will be settled after building first 8 ships. As it's completely new class there should be enough material from practical use to put the observations into practice (what works and what doesn't) and construct the latter 14 ships in a uniform manner. So the ships from 1 (2018FY) to 8 (2021FY) were set for installment of the VLS later.
However they included two sets of Mk.41 VLS (16 in the 2021FY supplementary budget so it seems like two ships of the "first batch" should be equipped from the launch. Those will be ships number 7 and 8. The remaining 6 ships will get their VLS in a few coming years. So it's coming earlier than it was previously predicted.