“The narrower you make Islam, the fewer the people who will fit into it.”
Shaykh Abdal-Hakim Murad
There’s probably two angles. Firstly, the takfir dimension. Intolerance of different opinions resulting in takfir of other Muslims. Secondly, a da’wah dimension. If you limit the expansiveness of Islam to a specific culture or time, without acknowledging its ability to adapt and thrive in different contexts, you will attract few new adherents.