BoringSSL caters first and foremost to Google's interests; LibreSSL caters to OpenBSD's interests. Either choice affected the final result in terms of what-can-run-where.

LibreSSL is mostly a drop-in replacement for OpenSSL, while BoringSSL has removed things that some applications will depend on. OSes shouldn't/couldn't replace OpenSSL for BoringSSL (the article says as much) but could replace OpenSSL with LibreSSL (some already have).

BoringSSL seems like the more ambitious project, LibreSSL the more pragmatic.