Because the for brings the whole file to the memory before walking thru the lines. This is usually slower than going line-by-line in the file (as the while does) because:

1. it fills many pages of the memory with the contents of the file, potentially swapping stuff out -- instead of just allocating a couple of pages for a file buffer;
2. then it walks thru those pages, potentially using cache lines -- instead of pulling the file buffer to the same cache line over and over, freeing cache lines to other stuff;
3. goes to the disk all at once, blocking until it has read everything -- instead of going to the disk when it had already processed the last chunk of information and then blocking for less time.