It's probably a good thing that time travel is not available. Just imagine the sheer number of people who would, in a heart beat, ditch this time-frame and move to another. Though I guess that would say it all about the present we live in..
But imagine the past flooded will millions, who knows even billions, of time travellers/tourists/refugees!!
Most I fear would act as pure imbeciles, either by trying to change things or adopting a sort of God-complex by drawing attention to themselves, or brandishing superior technology and their knowledge of the future. The degree of anachronism and irreversible contamination this would introduce could quite possibly collapse the space-time-continuum.
Scientific development needs to be in balance with spiritual and/or moral development for quite obvious reasons. Humanity, on the whole, is grossly out of balance in this regard. It cannot act responsibly in the present, let alone the past.
So yeh, for now it's a good thing that time travel is out of our reach.