Because Congress:
1. No longer has the capability. Without the facilities or the staff to enforce its subpoena power, Congress willingly ceded these responsibilities to the DOJ (who, if I'm not mistaken, also serves their subpoenae). Which makes perfectly good sense
2. No longer has the will. On that we can agree.
I remain unconvinced on the breadth of the Sgt at Arms' authority. It would make perfect financial sense for Congress to cede the enforcement authority (outside the local realm) and detention responsibility to the DOJ. At least so long as there was mutual respect for the prerogatives of the co-equal branches of government.
Here are three links to descriptions of the Sergeant-at-Arms Office and its duties and responsibilities -- two from dot gov, one from Wikipedia.
In none of these is the function of arresting and detaining anybody accused of a crime against the Congress so much as mentioned.
At one time, arrest and detention of such transgressors was clearly a responsibility of the Sergeant-at-Arms. These links suggest that may no longer be the case.
Issa is a joke, nothing comes of his hearings