1. Because its cheaper, I don't have any economic data in front of me just basic math.

avg 1st regional FO pay 25/hr x 1000 flight hrs/yr = $25,000/yr
avg max regional FO pay 43/hr " " = $43,000/yr

avg new CA pay (slide right across at year 5-7 pay) at a regional 70/hr x 1,000 hrs/yr = 70,000/yr
avg 10/yr regional CA pay " " = 95,000/yr

now mainline:

avg 1st yr FO $60/hr x 1,000 = $60,000/yr
avg max FO pay $150/hr x 1,000 = $150,000/yr

avg new CA pay (already on year 10-15 pay) $175/hr =$175,000/yr
avg 20 yr+ CA pay $250+=/hr = $250,000+/yr

Plus the work rules are much better, mainline work rules guarantee you more hours of credit with the ability to drop flying and often not lose pay.

Before I get torn apart the above numbers are just averages.

2. Liability, regionals are usually the starting point in the industry for most and hire the least experienced pilots. When/if something happens it provides legal separation, like Colgan 3407 or Comair 5191. Mainline gets them after they've been vetted a little bit. Hiring them at 5,000tt and 3,500 turbine instead of 1,500tt and no turbine. Reduces training failures as well.

3. Seniority, after spending 5-10 years at a regional the pilot moves up to mainline, 75% of the time they're around the 35-37 age range. But imagine if said FO started at flying a 50 seater for a mainline carrier at the age of 23 making $50/hr and building up all that seniority until retirement at 65 years old. He/she would top out at senior CA pay at 250/hr by age ~43 and stay there for 22 years topped out on pay. The current system restarts us at say? 33 so we top out 250/hr at 53 and only see that pay for 12 years or so.

These are a few of many reasons.

Even with regional pilots making more than in previous decades, our career potential is still a fractional of our mainline colleagues. Sucks.