From this source
Helium is used in scuba tanks to dilute oxygen. Why helium? First, it is chemically inert, that is to say, it does not participate in chemical reactions. Second, helium is much less soluble in water than many other gases, such as nitrogen. The low solubility means it does not enter the blood stream, even under pressures commonly experienced by deep sea divers.
Under water, a scuba diver is subjected to added pressure. In shallow water the pressure difference is not so great, and one can simply use compressed air to breathe. However, at a depth of 300 feet, a diver experiences nearly 10 times normal atmospheric pressure. Under these conditions, breathing compressed air can be harmful. The increased pressure requires an inert gas to be used to dilute oxygen.
Oxygen is essential for breathing, above and beneath the surface. However, one can get too much of it. Under high pressures, greater amounts of oxygen enter into the bloodstream. If the concentration becomes too high, oxygen poisoning can result, with symptoms that include confusion, impaired vision and nausea. So, under high pressures, oxygen must be diluted with some other gas.
Nitrogen is very abundant and relatively inert, but it cannot be used to dilute oxygen in scuba tanks. At the elevated pressures experienced by divers, larger amounts of nitrogen will dissolve in the blood causing nitrogen narcosis. Dissolved nitrogen also can cause a painful condition, called "the bends" if a diver makes too rapid an ascent. As the pressure decreases, the dissolved gas can form bubbles that can stop circulation in capillaries and damage the nervous system.
The wikipedia article on breathing gas says
Helium (He) is an inert gas that is less narcotic than nitrogen at equivalent pressure (in fact there is no evidence for any narcosis from helium at all), so it is more suitable for deeper dives than nitrogen. Helium is equally able to cause decompression sickness. At high pressures, helium also causes high-pressure nervous syndrome, which is a CNS irritation syndrome which is in some ways opposite to narcosis.
Helium fills typically cost ten times more than an equivalent air fill.
Equivalent air depth is used to estimate the decompression requirements of a nitrox (oxygen/nitrogen) mixture. Equivalent narcotic depth is used to estimate the narcotic potency of trimix (oxygen/helium/nitrogen mixture). Many divers find that the level of narcosis caused by a 30 m (100 ft) dive, whilst breathing air, is a comfortable maximum
I suspect earlier dives were not so deep , as the suits, storage and general technology were probably lacking. So a 2% was probably enough for the shallow dives.
Helium is a good diluting gas , but i think the new mixture was adopted to reduce costs as helium is very expensive.