They won't say for sure, but the city saidin the first issue of their two-issue run of "OnTrack" that it was due to cornering issues, since the train would have had to transition from 4th street to 3rd street before making it to Seaholm. And unlike Camden, our downtown isn't full of blocks with no economic value which we could cut a chunk off to make cornering more feasible.
And don't forget - even going all the way to Seaholm only addresses the downtown worker on Congress, and not particularly well at that, compared to running down Congress itself. It doesn't do one darn thing for the large population of potential additional transit users at UT and the Capitol, both of which were served directly by the 2000 proposal.
If you don't know anything about the 2000 plan, I recommend reading more of my crackplog - I cover the route in many places; the Crestview Station use cases is a good start - just think of the "direct route" as the 2000 light rail route, as I clearly indicated in the post. North of Lamar/Airport, the 2000 light rail proposal used the same right-of-way as commuter rail (although the 2000 proposal said they'd rip up existing track so the whole thing could be double-tracked, which could probably have been scaled back in a re-float).
I'm very familiar with the River Line by this point after Lyndon Henry's crowd tried to use it as an example of why it's not misrepresenting to call this line in Austin "urban light rail". I served on the UTC from 2000 through 2005; and stood on the street corner at 6th&Congress waving pro-light-rail signs at motorists on election night in 2000. I was one of the "anti" speakers at several events before the election in 2004, having to carry the entire load of the "pro-transit but we're about to build a bad rail line" crowd by myself as everybody else just bought the baloney about how central Austin would get served later if we held our noses and voted yes. Hell, I was on KXAN at one point, interviewed in my front yard.
So perhaps at this point you could credit me more than some anonymous jackass on this stuff, OK?