It is their acid nature combined with there very high consumption of water, which is a consequence of their very rapid rate of growth. Modern pine trees grow like inflating ballon toys. You can almost watch them grow overnight.

Old-school pine, like scots pine, are not so bad. But these are not grown commercially as much. Pity, it is a nice timber… more red than modern pine.

They needle down and kill the surrounding weeds with those needles. The shadow of the canopy is much more interlocked and closed. Its dark, dry and dead down there.

If trees are planted commercially, they’re often planted «too close» since not all of them survive to «old age». This makes them much more dense than what you’ll find in untouched nature. (That’s just one aspect, see other comments etc)

Pines shade the ground throughout the whole year, densely planted ones especially. Leave trees, like beeches and oaks, loose their foliage in autumn, thus in early spring other plants get enough light on the ground to flourish, before the trees produce their leaves again.

It's not the pines themself. It's when its only pines i.e. lack of diversity.

This is not why pines cause less biodiversity, but there is another reason why you tend to see less biodiversity with pines.

They are more prevalent in colder climates.