The reason is lack of ressources. Integrating them properly works well as long as there are enough ressources, but when too many come in a too short timeframe it sadly does not work (also makes it much harder for them to get proficient at German since they can live in their own bubble and just talk in their native language).

(And we have many ressources, but we (Austria) took the most immigrants per capita of all central European countries, even significantly more than Germany which is known for having taken so many. We really are trying.)