Upvote:1
While the days in Slovenia were a few more than in Croatia, it may have been viewed as marginal, as a separate visa for Croatia would be required (and that Austria, 2 days, were for making flight connections).
VFS Global in India visas for Slovenia application launch page notes Applicants travelling to Croatia for more number of days than Schengen must obtain Croatian Visa prior applying for Slovenian/Schengen visa.
Whatever the reason(s) for the visa refusal, your only option is to comply, if that is what is necessary for the planned itinerary.