Because variable names can’t start with a number. If variable start with number, making distinction between integer and variables becomes a lot more difficult
I myself have stepped into that when I started coding at codecademy. I think they just change this lst1 and lst2 naming to something else.
By convention python says that we SHOULD NOT use l (small case L) and I (captial case I) for naming objects (just the letter or maybe at the beginning of the name) as they can be misinterpreted as 1.
P.S. you can see that they both (l and I) look similar here too!
Note that the exact working indicates only single character variable names (like l, O, or I). The documentation doesn’t specify that multi-character variable names, starting with l, O, or I, aren’t recommended.
Because using l, O, and I at the beginning of a variable name can be a bit confusing, the documentation itself recommends using L instead of l if necessary.