The dialect of C++ was pure hell, and the wanton diversity of products meant that there was no chance to get consistent UI over a chock-full of models whose selling potential was unknown in advance. Theoretically, there were standards such as Series 60. Practically, those were full of compatibility breaks and just weird idiosyncrasies.
Screen dimensions, available APIs, everything varied as if designed by a team of competing drunk sailors, and you could always plunge a week of work into fine-tuning your app for a platform that flopped. Unlike Apple, there just wasn't any software consistency. Some of the products were great, some were terrible, and all were subtly incompatible with one another.