quite simple.....

 

u know abt conjugate acid and bases, right?? u also know that all try to reach stability .., right?/

 same here....

 

H2SO4 is a quite strong acid, so after losing one H+ , it becomes HSO4^- and it has more tendency to get one H+ rather than losing it...   on the other hand , water is aphoteric substance weak in both, OH- is a very strong base, so water has a tendency to form it. hence, its ka1 is more.

 

i hope it is  clear??