Three general points spring to mind:
- Ease of mapping from writing to sounds in Spanish. Spanish learners try to pronounce every letter, because that is infallibly right in Spanish. (It's interesting - and perhaps relevant - that when I was at a teachers' training session about dyslexia, one teacher reported that she had had a Spanish student [an adult] who had been successfully through the Spanish education system without having his dyslexia diagnosed .)
- L1:L2 phonemes almost all (?maybe all) don't match.
- Timing. See Timing - Wikipedia, the free encyclopedia)
PS: I've said a bit more about this here.