On the contrary, Kaine votes far-left almost all the time, and is consistently rated a zero or near-zero by conservative organizations like Eagle Forum or ACU. The media will portray him as some kind of a centrist bridge-builder, as they always do for liberals, but that he is not.
He made noises against the Iran deal back on the homefront, but in the end was cowed into voting for it, if you believe his reservations were genuine.
In the end, he's a generic face meant to appease WWC voters who is free of scandal so far as is known, which is probably all they wanted. The Clinton campaign is half about it being 'her turn', and half 'you have to vote for her because you won't dare vote for the other guy', so they didn't need anyone with baggage to distract from that.