why did the middle colonie depend more on indentured servants than slaves?

Morality aside, indentured servants made more sense economically than a slave.

You could easily buy a fully trained slave, but the slave was usually quite expensive. If you bought an untrained slave or one was born to your existing slave you had put the slave through a training period. During that time you got no work, and possibly some damages made by an inexperienced worker.

There were laws regulating the treatment of the slave, and once the slave was too old to work, or too feeble the owner had to feed and clothe that slave for life. By the same token, if you had dangerous work required, and the slave was hurt you had to care for him or her.

Slaves were forbidden by law and custom to be taught to read and write therefore they couldn't work as clerks or assist with any business transactions.

An indentured servant typically brought existing skills needed by the "employer." Although the servant may have had meals and clothing provided, it wasn't necessarily required. It all depended on the contract. The person taking the indentured servant could specify what he was looking for, and get it on his terms. Once the indenture was over, obligation was over.

Lastly, the middle colonies socially were more "do it yourselfers," religious and cultural norms meant that they typically wanted to work and do for themselves. If you were starting a farm or business you may need some help to start but once the operation was going, they wanted to do it themselves.

For one reason, the Quakers of Pennsylvania did not believe in slavery.