Miya Because there has been changes in trends both in China and the international audience. The tropes used from years ago are outdated and appear to be much more bland. As for recent authors, they have improved on that formula so they are a little more 'modern'. Those you read from years ago has the 'read one, you read it all' feeling. Some terms you see were memes of the time, but have lost popularity. For example, one of the terms is YY novels to describe these webnovels in general. That term is very seldom used these days. Depicting an outdated China is probably not in their best national interests (if any).
In addition, there is a thing about copyright. Qidian doesn't own the copyrights forever. (About 10 years). So if they translate an old book, the copyright to the old book might run out before the translations finish, etc. There is always the extension of copyright which the author sells it again which apparently most do, but there's too much risk in that.
Some novels that have been picked up by other independent translators might not be that popular in China, so there's little incentive to promote them either. Generally, books that aren't popular in China are not written as well because the author gets disheartened at the poor results. They might be ended abruptly among various reasons. Qidian has the data that says which books are popular, so it's a safer bet to choose the ones that were popular in China, as there is a higher chance of them being popular to the international audience. This is evident from the very popular webnovels that were previously translated.
Just some reasons I can think of.