I chock it up to the white lotus being terrible teachers. The Avatar is meant to travel the world and learn bending from various teachers and build-up a connection with the world, both physical and spiritual. Which is where I believe is where alot of the power of the Avatar state comes from. Aang did alot of that both pre and post iceburg nap. Korra has only really started, which was then halted for 3 years because of Zaheer.
I think you need to work for it to get anything from being the Avatar. Korra, got most of it handed to her, including her Avatar state. Aangs ghost decided to just give it too her when the ability should be earned.
Really we can blame Aang overall for Korras lesser ability Physically, Mentally, and Spiritually since it was his idea to seclude his successor until the lotus felt her ready. She wasn't fully prepared for how scary and complex the world is.
That's my interpretation of it anyway.
Since once Korra started getting out of Republic City in Season 2 she started getting better.