Because there are legal matters and also costs involved :-

Legal Matters
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They seem to believe that they have to sent letter after letter, of different colors, before they can finally cut the supply. That usually takes at least 3 or 4 months. The average users can easily blow the usage above several hundred % of the deposit.

Costs
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There are costs involved when sending people to cut the supply. And when the technician reached there, he often got bribed by the tenant and so the supply is still not cut.