"so why they were not keeping an eye on him" - good question. It's now revealed that Ayoub was known to authorities in France, Belgium and Spain. I can only guess that there are now so many people who the authorities are attempting to watch that they can now not watch all of them, all the time.
What strikes me as odd is if he is indeed someone with strongly held beliefs and was attempting to commit a religiously inspired attack, why did he try to defend himself and say that he simply found the weapons 'in a park' near the train station. If you love your God so much you want to kill people, at least be ballsy enough to say so, and not turn to jelly and deny it when your plan fails. Doesn't sound like he is that religious to me. At least the Al Qaeda guys have the courage to say what they truly believe.