Justice departments notably have trouble keeping up with modern technology. Where I live, it's still impossible to get a digital copy of your file (leading to a case where someone ate an important document and was able to go free on a technicality).

Not just that, but smartphones are not quite ubiquitous yet. Either you require the person to purchase one, or have the state purchase one, neither of which is ideal.

I suspect there are also legal and human right problems, since ankle monitors are already used as a form of punishment, but have never been used (as far as I know) as a parole/probation measure.