Because in the early 2000s Open office was given a fuck load of publicity by sun and then oracle. As such it is still the first name many less OS informed think of when they think of an open source office suite.

Libreoffice has simply never had that sa,e level of publicity.

It is still to this day a fucking fight to convince many who look for an alternative to Microsoft. That Libreoffice is the true OpenOffice replacement.