Why can't we propose a solid theory regarding origin of life?
First of all, we do have some solid theories to explain the origin of life. That said, the main thing missing in order to be able to accept one of these theories with a decent amount of certainty is the ability to run a planetary scale experiment for a few billion years.

The theories are fine, the problem is that we cannot really test them. In order to test a theory that describes the origin of life, we will have to recreate the original conditions (think of a test tube the size of the Earth) and then pump energy into the system and watch what happens over the next few billions of years. This is not really feasible, for obvious reasons. I really doubt you will be able to get funding for an experiment that will run that long.