In reversible process,due to infinitesimal steps,the system remains in thermal equilibrium(why?) . But in irreversible process, thermal equilibrium does not exist. What is the cause for this phenomenon? Why the former attains thermal equilibrium and the later cannot? Plz help me explaining this.
What your textbook says is that during the course of your process, the system is in thermodynamic equilibrium with its surroundings if it's reversible. Such a process is just a theoretical concept as such a process would take an infinite amount of time. Real processes (I mean irreversible processes) take place so quickly that it is impossible for it to be in thermodynamic equilibrium (i.e. in Mechanical, Chemical and Thermal equilibrium) with its surroundings at every step of the process. But the analysis of such a process becomes impossible as we cannot exactly pinpoint the non-equlibrium states that are passed through by the system between the initial and final states of the process and so, we approximate this process by an adiabatically slow one, and define a concept called a 'Quasi-static' process which is the locus of all the equilibrium states passed through by the system in the course of your process.