Why is the index not used?
Your index probably only contains the lastModified column, as well as your _id value as a way of finding the other column values for each row.

So if your predicate (the WHERE clause) isn't filtering your index data down to few enough rows, the effort in looking up the other details over and over is too much and it will decide that it's easier to just scan the whole table.

If your index included all the columns you were interested in, you would see an ordinary seek.