The original git had a command line interface. It's widely assumed that using a GPL'd program in your program through the command line does not cause the GPL to "infect" your program.

OTOH, one of the major reasons for grit is to provide a library interface. If they kept it GPL, anything that used grit through the library interface would have to also become GPL.

This could be the "legitimate concern" you're asking for.

But the LGPL was also an option -- it addresses that arguably legitimate concern and keeps the spirit of the original license.