1. FLAC files are great, but they are huge.
Even on today's devices, FLAC collections can take large amount of available space.
2. FLAC files are great, but they have compatibility issues. Many devices have issues with FLAC files - stutter, gaps, various buffering issues, slow load times and, in worst case, some devices are not able to play them at all.
3. FLAC to MP3 converters are usually not free or are too complicated to use.