It's very possible they just decided it wasn't worth applying for 501c3 status. I don't think they are prohibited from it but I think there can sometimes be nuances that complicate things like a requirement to pay for expenses if funds aren't raised that could prevent them from obtaining it because it creates a scenario where they could directly benefit from the fundraising.
As far as sponsors go, I doubt corporate sponsors would be deterred as it is an expense for them either way, regardless of whether they treat it as charitable giving or advertising.
Best,
John Smilde
Director of Development Operations | Cato Institute
1000 Massachusetts Avenue NW, Washington, DC 20001
jsmilde@cato.org
202.216.1451
B