Redshift and mirror diameter. There is very little old light, so to see further, you have to collect more light to have a chance of catching those ancient photons. Also, due to expansion of space, the older the photon, the less energy it has, so you have to look deeper and deeper into infrared.

Disclaimer: not a cosmologist. We can see back further with other instruments, just there's less to see.

The universe's history: https://en.wikipedia.org/wiki/Chronology_of_the_universe#/me...

- We can see the cosmic microwave background (CMB), the earliest photons after the big bang still observable. This is ~14 minutes into Jan 1 if the whole age of the universe is a year. Satellites like the WMAP have done a great job of that.

- The dark ages that follow had few photon sources.

- JWST will be observing the earliest stars following that era.

There's a proposed radio telescope (that would have to be on the far side of the moon!) to observe neutral hydrogen photon emissions from the dark ages: https://en.wikipedia.org/wiki/Dark_Ages_Radio_Explorer

There were maybe 100 million years after the big bang with no star formation. Darkness. Here's a page about that era as it relates to Webb: https://webb.nasa.gov/content/science/firstLight.html

> what prevents us from seeing back any further?

The other comment mentioning darkness is wrong.

In the very early universe, it was extremely bright and hot. It was only after 100,000 years or so the universe cooled down enough to become transparent.