Welcome!! We simply don't have access to all the evidence the prosecution and police have gathered. I am 100% positive that they did not arrest him without a solid case - I have no doubt more details will come out when the case goes to court.
Also, many murders and other crimes happen where the person was not seen at the crime scene, was not seen committing the act, etc, but a conviction is possible even with circumstantial evidence. I personally believe, even on the little we know so far, that the evidence would be strong enough to stand up to any jury. Guess it really comes down to what evidence is allowed to be presented in the court, what members are retained in the jury, and how good a job the prosecution and defense both do.