Why god cannot heal amputees? Well... he did, once.

[Angrboda]

An article at skeptoid has an alternative explanation. ()

One key detail they note is that there is no testimony of anyone having examined the amputated leg to confirm that it was gone. There is no record of him being seen at the hospital at Zaragoza where the leg was supposedly amputated, and the amputated leg itself which was supposedly buried is not in the cemetery where it was supposedly buried.

Skeptoid's alternative explanation is that during his 50 day convalescence with a broken leg, he was forced into begging to earn what money he could. Towards the end of his convalescence, he realized that if a broken leg evoked sympathy and money while he was begging, a fact which wouldn't last forever, then a missing leg would be even better. So he left the town where people knew him and headed out for a strange town (Zaragoza), and, binding the one calve behind his thigh, made a living as a one-legged beggar. Of course, he wouldn't sleep with his leg bound up this way, thus explaining why the existence of his supposedly missing leg was discovered while he was sleeping and by a third party.

Do we know that this is what happened? No. We can't confirm the alternative story, but it is fully consistent with the evidence, and seems more plausible. This is a fact which the religious repeatedly fail to take into account when judging the likelihood of a miracle: people lie and make up stories. And not just under extreme circumstances; they do it all the time. So given that fact, the question is how likely is it that event X occurred relative to the probability of lying or invention, not just how improbable is the event relative to natural occurrences. The probability of dishonesty or confabulation, if consistent with the evidence, is always going to be greater, which means that, unless you can rule it out with concrete evidence, that explanation is always to be preferred.

(ETA: Note also that the weakened condition of the leg after discovery is consistent with the atrophy and other effects of living with a leg bound to one's thigh in that manner.)