"I keep hearing people say this and never backing it up with facts. I know renewables have their own environmental issues but why should they be a show stopper? "
Because they can't supply baseload.
(Geothermal is NOT a renewable. History shows that the extracted heat tends to be pulled from the ground faster than it's supplied from below - Iceland is one of the very few exceptions worldwide - and they also have significant environmental impacts due to highly polluted groundwater entering the ecosphere.)
With the best will in the world, until every large renewable source is piggybacked by a battery bank, they never will be able to supply baseload either - which is why utilities are refusing to connect them unless forced to, or are paying renewables generators to NOT connect.
If the amount of subsidy poured into solar since the 1970s had been put into LFTRs instead we'd already have a large fleet of nuke plants which are significantly safer _by design_ than the Heath-Robinson (Rube-Goldberg for your USAians) contraptions currently deployed and being built and, would produce at least 98% less high level waste than those current designs and be a LOT harder to extract any form of bomb-making material from too. On top of that, because they don't need to run on enriched uranium, the environmental (and carbin) impact of mining for fuel is vastly reduced, let alone the energy requirements of enriching and the substantial wastage of potential fuel at that point (250 tons of mined uranium becomes 140 tons of enriched uranium and 90 tons of depleted uranium, both of which can actually be used for fuel but it's much easier to mine thorium and feed that to a LFTR instead)
Instead, the only existing LFTR plant was last run in the 1960s and shut down by presidential order in 1972 because it couldn't produce plutonium.