It's harder to develop features, because you have to get a feature to work in multiple runtimes instead of just one. So the language might be developed at a slo

It's harder to develop features, because you have to get a feature to work in multiple runtimes instead of just one. So the language might be developed at a slower pace. You're also likely to run into bugs where something works for CLR but fails on the JVM (or vice versa). So more energy has to be spent fixing mistakes versus features. Then there's fracture. Imagine reading that language X is at version 3 for one runtime, but 2.5 for another runtime because "the guy who did the support of runtime B lost interest, and nobody has taken it up again".