The question has kinda been answered by the above posts.
T-props of the hay day were excellent aircraft. Viscounts, DC-7s, Vangaurds ect. Some of these aircraft were the fastest T-props in the world until the Q-400. Not only were they fast, but they had very good operating economics. As such, there profit potential was very good. Lets compare 3 types of T-props through the ages:
Vickers Viscount 810:
Seating: 65 (75 max)
Fuel: 640 kg/hr
Speed: 310 kts
Fuel: 590 kg/hr
Speed: 270 knts
Fuel: 760 kg/hr
Range: 1510 nm
Speed: 350 kts
Judging from the numbers, the Viscounts numbers stack up pretty good for a 40 yr old designed compared to the Q-400 eh? In some respects, the Viscount scores better then the 210, an aircraft produced 30 years later.
Its because of this Viscounts are the core aircraft in a number of big airlines across the world in JA#3.
Excellent operating economics + strong profit potential = a very good aircraft.
It doesn't hurt that you can turn a profit @ 30% LF too.. think you can do that with a 707 or DC-8?
Aircraft selection plays a major role in your ability to expand in the beginning of the game. Some aircraft are better then others for starting. At the start, you can have 2 707s or up to 7 Viscounts. Now, which airline will make more cash flow? 2 aircraft or 7?
Hopefully this helps