Simplify the following text for a Grade 8 native English-speaking person.
Original: Arab-plan mosques were constructed mostly under the Umayyad and Abbasid dynasties ; subsequently , however , the simplicity of the Arab plan limited the opportunities for further development , the mosques consequently losing popularity .
Simplified: Arab-plan mosques were built mostly during the Umayyad and Abbasid dynasties . The Arab plan was very simple , which did not allow for much further development .