It's not. This has been the policy for a couple of years.
Every major sport event, concert, etc., nationwide, has had this policy since the Las Vegas shooting in 2017. Concerts and other similar events have had this policy for a couple of years, too. Note that Target Field is just following the MLB policy.
One pandemic and we forget stuff, I guess.
You are not remembering correctly. Also the event that kicked off the metal detectors and so on was the Boston marathon bombing. The Vegas shooting didn’t impact anything unless it took two years to implement which I doubt since it took less than one year for metal detectors to be put in after the bombings
Here is the policy that started 07/2019:
Allowed: small purses, totes, clear plastic bags or a tote bag, Small soft-sided coolers, diaper bags. Maximum bag size is 16 inches by 16 inches by 8 inches.
Not allowed: backpacks, laptop bags, bags with a lot of zippers (such as non-clutch purses), duffel bags
the beginning of the 2019 season and all seasons before, backpacks, laptop bags, lots of zippers and big bags (as long as smaller than the size listed above) were allowed.
2021 policy allowed: medically-necessary items, diaper bags and small clutch purses (max. 9” x 5”) only. Everything else, banned.
So this is a significant change even from the July 2019 policy which is the last policy that had fans attending. I’m pretty sure this is implemented this way this year because of COVID. They’re trying to make everything as contactless as possible.
Whether this continues beyond 2021 remains to be seen. MLB has always been the most lenient on what is brought in compared to other sports. COVID may permanently change that. We will just have to see.