They do this because although they rate incumbents, the Libertarian Party makes no effort to determine which incumbents deserve to be opposed and which ones deserve to be supported or to control which races they run candidates in. To some degree this is because most of their candidates are just paper candidates who aren't even campaigning. They just sign up and get nominated with no oversight or even basic assessment of their qualifications. This is a terribly misconceived strategy which degrades the quality of the Libertarian Party brand and can lead to the defeat of sitting libertarian Republicans in hotly contested races where the 1 or 2 percent vote for a Libertarian Party candidate can do real damage.
It's actually rather bizarre that the Libertarian Party in Texas would do all the hard work of rating candidates and then release the ratings too late in the election to be much use and without making any effort to use them as the basis of how they target their efforts. If they focused on running just a few well funded candidates against the worst legislators and didn't waste so much time and money on unelectable challengers to Republicans with libertarian voting records, they might actually win some races and earn some respect.
As things stand now, the release of these ratings just shows what a sad joke and what an obstacle to the advancement of libertarian ideals the Libertarian Party is in Texas. Their misguided efforts are likely to reduce the number of libertarians in office in Texas largely out of an irrational obsession with party labels.