I’d say it’s probably a combination of factors. There may not have been enough early voting sites, but it was also the first day, so more people than average may have been trying to take advantage of early in-person voting.
The lack of sites may also reflect the general underfunding of our election system, which is a long-standing problem. But with the challenges of running an election during a pandemic, this has made our electoral system’s shortcomings even more apparent.