The explanation is simple: raspberries give shoots at very extended distances and grow a huge root system. When planted close, the roots of the bushes will become intertwined and you will not know which variety germinates.
This is especially bad if one of the varieties is remontant, because you can mix up the plants, cut them incorrectly and thus lose a large part of the harvest. To avoid this, be sure to plant different varieties at a distance of at least 2.5-3 m from each other or dig in partitions up to 50 cm deep between them.
For example, you have planted raspberries properly, but they look worse from year to year. The bushes are thinning, the berries are getting smaller and the taste of the harvest is not at all pleasing.
The place for this crop is usually chosen by residual principle, and the same thing goes for care. Raspberries need regular fertilization, otherwise, having exhausted the soil, they will weaken, and this will primarily affect the harvest. If you do not want to suffer damage, be sure to apply organics under the bushes in autumn, and complex fertilizers in early spring and before flowering.
Another reason for deterioration is diseases and pests. If the plant wastes effort on fighting the disease, it has no strength left to produce crops. Therefore, do not forget about regular inspections of plantings and treatment of shrubs.
In particular, it may happen that the varietal raspberries you had planted at first died a long time ago, and in their place a wilding grows, which impudently “came” from a neighbor. Then the harvest should not be expected either and you need to remove all the shoots until such raspberries have occupied all available territory.
Raspberries are a crop which does not require much care and is very “grateful” for proper care. It can be brought to deterioration only as a result of complete indifference and non-compliance with agricultural techniques.
Take care of raspberries and then any varieties on your plot will look exactly like in a nursery of professional farmers.