It's a trend, away from the plastic spools which were decidedly made from a non-recyclable plastic.
It's always been curious to me that recyclable plastic was never even considered to be an option.
That said - cardboard is recyclable and biodegradable, so it's good no matter how you dispose of it.
But - it does have environmental downsides. If not carefully sourced, the wood can come from places where deforestation is a problem. It's also rather energy intensive to make, so depending on the energy source can contribute to CO2 emissions.
Nothing will be perfect, but cardboard is likely better than the alternatives.