He's given that much because he makes the least amount of money in all the heists. By the time you switch to him, Mike and Frank already made $400k+ from the first heist and like Lazyboy said, after purchasing the McKenzie Field Hangar, he's left with very little while Mike and Frank still has a sh*tload of money.
The whole point of being given 150k is to buy the McKenzie Airfield Hangar.
Both of these. One of the first things I did with Trevor was buy the Field Hanger and get in business with Oscar. If you buy it right away, it leaves you with around only a couple grand left, thus enticing you to do these missions.
Also, as mentioned, by the time you're introduced to Trevor, you've already done the jewelry heist with Michael/Franklin, and between the two of em, are sitting on around a million bucks. Even though it forces you to be Trevor for a few missions directly after the heist, once you complete them, if Trevor had only around $5,000, while Franklin and Michael both have six figures, it may dissuade you to be Trevor as much. The average person would probably be drawn to using characters with solid spending money on guns/cars/clothes/stocks, and ignore a broke Trevor. Just my 2 cents anyway.