Because, unlike any other President in the last 45 years Trump did not disclose his tax returns willingly. This law was created because of what happened 98 years ago in the Teapot scandal and was created for exactly this sort of situation. this is literally what the law was created for.
And BTW this law is not all that obscure, Republicans used it just a few years ago to get the tax returns while investigating if the IRS was discriminating against conservative organizations. It was not controversial then, why is it now?
And they don't need to. Legislative purpose does not mean 'to make a law' it means 'a valid purpose of the legislature' and oversight of the President is a valid purpose of the legislature. This was decided by the SCOTUS already.
There is literally no legal argument to believe that. SCOTUS came up with the term legislative purpose in a similar case, and decided that Congress can do this. This has already be adjudicated, there is precedence.
And BTW, we can expect to find direct illegal activity on Tax Returns. That is what happened to Nixon, causing him to utter the now infamous words 'I am not a crook'.