I think that is small-think. The technical solution is only part of the problem and scaling up all systems to meet the .1% case seldom makes sense. They were smart to save 5-10%.
So, probably like most software of this nature, the reason it's not scaling is simply because the people who made it probably weren't the greatest engineers on the block.
Consider: single auth across all the state's services, external APIs, identity verification, address verification, employer ID verification, federal/military ID verification, income/tax verification, phone verification, bank account information, translation into multiple languages, accessibility features, etc. Also, there's probably a lot of legacy infrastructure and process.
Also, if "ability to burst to 10x normal filings per week that might happen once every 40 years" wasn't in the spec, I think they were right not to engineer for it.
It sometimes borders on sealioning.