The short answer is that Russia knew Trump was much more likely to give them what they wanted from us. They interfered in our election just as we have interfered in dozens of elections in other countries over the years, and for the same reason. Anyone who understands the US's recent history would understand that completely.
Russia has offered help to MANY candidates through the years, in fact al through the Cold War. Adlai Stevenson (who ran twice against Eisenhower in the 50s), Hubert Humphrey, and others, received communications from Russia offering help, and they reported them to the FBI. Trump, OTOH, accepted their help. The meeting with Veselnitskaya in Trump Tower offers pretty good evidence that Trump knew they were helping and accepted, and even coordinated their help.
Trump had a deal working in Russia to build a big Trump Tower. It was the biggest business deal in his career so far--so profitable that he offered a multi-million-dollar penthouse apartment to Putin for free. My guess is that he also offered to lift sanctions, the Magnitsky Act, to help Putin and his mob use American banks for money-laundering. We can't prove (yet) that he offered this to the Russians, but he said to US many times that he was going to lift the sanctions.
After the sanctions were put in place, Russia retaliated by banning American adoption of Russian children. So when Don Jr. got caught, he said (at first) the meeting was about adoption of Russian children. Because when his dad lifted the sanctions, Russia would lift the ban on adoption. How clear could he have made that?
NO Democratic candidate would have lifted the sanctions. They were passed by an almost unanimous bi-partisan vote in both houses of Congress, and how often does THAT happen? Trump seemed to believe he could overturn them just on his own. Trump never really understood the constitution's definition of the job of the president, and he never had the attention-span for anyone to explain it to him.