In general, why is it NOT a good idea to estimate the testing effort based...

The quality of the development estimate may be poor.

In general bottom-up estimation is always better than top-down estimation.

The percentage based technique only applies to the V life cycle model.

Using the same percentage every time does not address the level of risk of the application to be tested.

The maturity of the organization, e.g. the quality of the test basis, quality of development testing, configuration management, availability of test tools, also influence the effort needed for testing.

It builds on large set of historical data

The result is almost always a too low estimate for the required test effort