Because at one point you have to drop support for cards to be able to introduce new features without having to fight with legacy stuff. Otherwise you’re codebase becomes unmaintainable. They decided that the supported hardware is current DX11/OpenGL4.3 (and newer) hardware, which they introduced with Fermi (400 series and up). This is still 5 year old hardware (which is half a millennium in the IT world).
Fermi, Kepler, Maxwell architecture are hardware wise probably a lot more similar than Fermi compared to the GTX 200 series (Tesla architecture wich was introduced in 2006 with the 8000 series).
Exactly what blackout24 said. They have not abandoned those cards, they are still fixing bugs and updating the 340.xx driver branch in Linux to maintain the usability of those cards now and in the future. The latest driver for your card is 340.76 in the “legacy branch” and was released on 1.27.15, so not that long ago. In order take the newer generation of DX11/OGL4.3+ cards forward they needed to drop support for the older ones in newer driver branches.
They have done the same exact thing in Windows too, so dropping Linux for Windows won’t make any difference. The latest 347.88 windows drivers do not support anything older than Fermi(400 series). The current driver supported for your card in windows is 341.44 released in feb. of this year. They will continue to maintain that 340/341.xx branch of windows drivers for the old Legacy cards as well.
So this isn’t really a bad thing, your card will continue to work on the legacy drivers that will continue to be maintained, and the newer cards are able to be better optimized for by dropping the old code that supports the old cards.