Shit happens. Inter is king in Seria A this season, but not in UCL, because UCL is for champions from various leagues in which every club is very strong. This is why you will see that a club can be strong in his domestic league, but will not be able to go far in UCL. So many clubs like that.
However, been eliminated in the Champions League does not mean that they should be unhappy with such lost. They still have the trophy to win and become the defending champion of the league next season. They will only need to work on themselves and prepare for next season so that they don't get kicked out so early from the champions league.
Inter Milan been kicked out of the Champions League competition could be as a result of decided decision of focusing much on what is within their reach against what they can only try and not win. What I mean is that, it was crystal clear that even if Inter Milan exceed this stage in the Champions League, they could still be eliminated by a bigger team before reaching the finals. Since Serie A is still within their capacity and can confidently fight and win it, they focused more on it and I would say they will get what they desire by end of the season. Inter Milan would be crowned as the winner of the Scudetto this season and this is not debatable as they deserve to be given that title after coming this far to fight for it and made a lot of sacrifices.