The supreme court justices are more willing to declare acts of congress unconstitutional than to strike down presidential actions as contradicting the u. s. constitution. why is this the case? when have the supreme court justices checked presidential power in the past?

Laws passed through congress have a direct impact on the court system, since it changes the way courts have to rule on the law. The Supreme court allows the court system to have some say in what laws are just by appealing their agreement with the constitution. The President doesn't directly pass laws, he has the power to veto congressional laws and through his endorsement behind them, but doesn't actually have the power to write, create or pass new laws himself, even if he's the one who technically signs them into law. As such, the supreme court checks the president less often than congress, because the president's actions affect the court's sphere of interest less often. Most interaction between the president and the court happen when the President heavily endorses a bill, gets it passed through congress, and then the court checks it. Some great examples are the Agricultural Adjustment Administration and the National Recovery Administration, which were created through bills sponsored by Franklin Roosevelt as part of his New Deal reforms. The court struck them down as unconstitutional for various reasons, much to the dismay of FDR. In modern times, Obamacare almost had it's individual mandate requirement stuck down by the court a few years ago and elements of President Trump's muslim travel ban were struck down by the supreme court just in the last month.