<i>Short answer would be availability of tagged data,</i><p>Part-of-speech and syntax-annotated corpora have been available for a long time for many other languages than English. E.g. for German:<p><a href="http:&#x2F;&#x2F;www.ims.uni-stuttgart.de&#x2F;forschung&#x2F;ressourcen&#x2F;korpora&#x2F;tiger.html" rel="nofollow">http:&#x2F;&#x2F;www.ims.uni-stuttgart.de&#x2F;forschung&#x2F;ressourcen&#x2F;korpora...</a>
<a href="http:&#x2F;&#x2F;www.sfs.uni-tuebingen.de&#x2F;ascl&#x2F;ressourcen&#x2F;corpora&#x2F;tueba-dz.html" rel="nofollow">http:&#x2F;&#x2F;www.sfs.uni-tuebingen.de&#x2F;ascl&#x2F;ressourcen&#x2F;corpora&#x2F;tueb...</a><p>Dutch:<p><a href="https:&#x2F;&#x2F;www.let.rug.nl&#x2F;vannoord&#x2F;Lassy&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.let.rug.nl&#x2F;vannoord&#x2F;Lassy&#x2F;</a>
<a href="http:&#x2F;&#x2F;lands.let.ru.nl&#x2F;cgn&#x2F;" rel="nofollow">http:&#x2F;&#x2F;lands.let.ru.nl&#x2F;cgn&#x2F;</a>
<a href="https:&#x2F;&#x2F;www.let.rug.nl&#x2F;vannoord&#x2F;trees&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.let.rug.nl&#x2F;vannoord&#x2F;trees&#x2F;</a><p>French:<p><a href="http:&#x2F;&#x2F;www.llf.cnrs.fr&#x2F;en&#x2F;Gens&#x2F;Abeille&#x2F;French-Treebank-fr.php" rel="nofollow">http:&#x2F;&#x2F;www.llf.cnrs.fr&#x2F;en&#x2F;Gens&#x2F;Abeille&#x2F;French-Treebank-fr.ph...</a><p>I think that one of the problems is that NLP research of English typically has a higher chance of getting accepted at conferences.