The actual reason for why you feel the need for reification is because the type systems of both C# and Java are so poor that you end up in situations where you really need to do "isInstanceOf" checks and castings. But people are missing the forest from the trees here, because these are essentially holes in the type-system, only needed because the languages are not expressive enough. Because really, you don't really need to check whether something is an instance of "List<int>" at runtime, in a static language, unless your language sucks ;-)
As to why type erasure is among the best features of Java, that's because with reification it gets hard to support other languages. It's kind of ironic, but .NET CLR was once marketed as the multi-language runtime, but time has been on the JVM's side in that regard.
For example F# doesn't support higher-kinded types because the CLR doesn't support higher-kinded types and introducing support for HKT would mean for the F# compiler to basically do type erasure by itself, which would place it at a serious disadvantage versus the host language in terms of performance. Scala supports higher-kinded types, which means you can express in Scala types such as the Monad and the Applicative, which are essential for FP in static languages, without sacrificing performance or Java interoperability. Even OCaml supports higher-kinded types.
And this isn't just about static languages. Pick any dynamic language you can think of, take your pick from Ruby, Python, Clojure and Javascript and compare the performance of the implementations on the JVM versus the CLR. OK, it's sort of an unfair comparison, given the JVM implementations have had more resources invested in them, but you know, one of the reasons for why dynamic languages work better on top of the JVM is because the JVM's bytecode is basically dynamically typed, with the last barriers being brought down along with invokedynamic. But by introducing generics reification on the other hand you end up with more static info in the bytecode that dynamic languages have to work around.
The only clear advantage to reification is the specialization for primitives that the CLR is doing for things like the standard collections. But you know, this can be a job for the compiler, you don't actually need the runtime to do specialization (see Scala's miniboxing or Dotty) and even if the runtime does specialization, you don't need it to do .NET-like reification (at least the Java engineers claim that, having plans to do primitive specialization for Java 10 or something).