The Atari 400/800 home computers were well-established and quite successful in the games market by the time the Atari 5200 console was released in 1982. The 5200 shared the chips and architecture of the 400/800, and it appears on the surface that it would have been a trivial matter to have the 5200 support games on cartridge that already existed for those machines. In fact, those game cartridges likely comprised the best quality home computer game cartridge library that existed in 1982, for any machine.
We know in retrospect that the 5200 was not a success. And it seems like launching with a whole bunch of high quality games would have been really helpful. That, and a better controller, would have probably saved the 5200 from mediocrity. We also know in retrospect that forsaking compatibility with the 8-bit computers was a mistake in Atari's mind as well, since they corrected this deficiency with the XEGS.
Why did Atari forsake compatibility with the existing 400/800 game library?