Spam/excess data effects node runners. Node runners are the people who voluntarily run nodes to enforce the rules of the network, without them you don't have decentralized enforcement of bitcoin rules. More data means more storage space required on the node. Keeping it smaller allows the average person to run node, right now we're creeping over 1TB so those who haven't moved to 2TB will need to upgrade. SSDs are the best option, but they aren't cheap and they blowup every so often. If you want to go all out and protect your node data then you need 2 for RAID and that's gonna get pricey.

I still don't quite get why this extra data can't be put on an L2 that people can choose to run or somehow linked to the main timechain instead of directly on it. It seems messy with people just triyng to jam it in instead of getting a better solution.