"Building trust" is an abstraction that covers many many activities. The fact that Russia did many things that could have built trust but didn't is completely lost on you, so you have no ability to question WHY trust wasn't built as a result of the actions taken. Because if you DID question why, you would see that Ukraine's transition to a right-wing Euro-centric government entailed it being Russophobic and part of the European project to dominate Russia.