In Twelfth Night what makes Sir Andrew and Malvolio so easy to trick?
To put it simply, Malvolio and Sir Andrew are easy prey for the likes of Maria and Sir Toby because they recognise the failings of these characters and their susceptibility. Sir Andrew is obviously presented as a fool in Act I scene 3 in the discussion that Maria has with Sir Toby, yet it becomes obvious that Sir Toby is bleeding Sir Andrew of all of his money, using it to party at Sir Andrew's expense, by flattering his pride by suggesting that he has a chance of being successful in courting his niece, the Lady Olivia. Note what Sir Toby says about him: "Why, he has three thousand ducats a year."
In the same way, Malvolio's failing is his own pride and sense of self-importance. Note what Olivia says to him in Act I scene 5:
O, you are sick of self-love, Malvolio, and taste with a distempered appetite.
It is the recognition of this failing that makes Malvolio such easy prey to the idea that he has a chance with Olivia, and that she is, in fact, madly in love with him. Thus, the susceptibility of Sir Andrew and Malvolio to being tricked lies in their own failings and the ability of Sir Toby to recognise those failings and exploit them, with hilarious consequences.