Gentoo Linux Becomes a Software in the Public Interest Project Because…Bookkeeping and Taxes

why are they ‘going away from being a 501c3?

You've got it backwards. They're dissolving their old non-501(c)(3) Foundation that they use to manage their finances currently, and becoming a part of an existing 501(c)(3) called Software in the Public Interest (SPI)

TL;DR from the article, they're doing this to:

1. Offload the overhead expense of independently managing their finances/accounting, by merging with someone who already has a robust system in place
2. Save money by becoming tax exempt
3. Allow themselves to accept matching gifts from donors whose employers support such things

Reading between the lines on the gentoo-dev mailing list, I gather that the old system just was not working very well, with friction between the Foundation and the technical side of the distro.