Forst of all, while impeachment may be a politicized process, from a purely evidential standpoint it isn’t clear that Reagan was aware of and authorized the diversion of funds from the arms sale to the Nicaraguan Contra rebels. (Most historians believe he did and was insulated by Weinberger, McFarlane, et al.) The sale of weapons to Iran via Israel, while against stated policy of the Reagan Administration, ethically suspect, and geopolitically bizarre, did not comprise a criminal act. However, Congress added amendments to the appropriations bills specifically prohibiting providing aid to the Contras (the Boland Amendments), which was the mission proposed by John Poindexter and managed by Col. Oliver North using Southern Air Transport planes. This was a blatantly illegal operation run using CIA assets but without being formally authorized or managed by the Agency.
A bunch of people were convicted but pardoned by George H W Bush while appealing their sentences, and Oliver North’s conviction was overturned on Fifth Amendment grounds. Nobody in the administration served a day in prison so expecting to justify impeachment and conviction of Reagan, especially in the waning months of his last term, would have been an exercise in futility. It also would have been disruptive to relations with the Soviet Union as the Warsaw Pact began to crumble, so there were practical reasons to not pursue the quixotic course of impeachment.
Personally I think Reagan was aware at least at a policy level even if not of the mechanics of the operation or the shady contacts involved in the arms deals and banking manipulations, and they policy makers who actually led the operations should have been held accountable for not only violating Federal law but (knowingly or otherwise) facilitating the very drug trade it was so aggressive in prosecuting domestically. But opinions differ.