I'm not sure you can say that colonies were popular with the lower classes (by the way, "plebs" does not refer to the lower classes exclusively there were rich and powerful plebians and poor patricians).

Anyways, land reform like denying inherited claims to public property and founding colonies was considered "populist" in Ancient Rome but that doesn't mean that its exactly what the urban poor wanted. More and more scholars are debating exactly what the Gracchi and other reformers thought a bunch of urban poor were going to do with a little land.

Two examples: Sulla gave land to his soldiers after his wars with Marius. He did this by taking territory from troublesome cities in Italy (Pompeii, for example). However by Cicero's time Sulla's veterans had already sold all their property and were living in heavy debt. They made up Catiline's largest support for his campaign to absolve debt.

Next is Clodius. Of all the politicians Clodius is probably the most popular with the urban poor. He could command riots and had a large gang of toughs around him 24/7. He made a lot of populist reforms but land reform was an afterthought. He expanded the bread dole and allowed people to meet in collegia again. Those were organizations that acted like neighborhood housing associations, unions, and hiring bureaus all wrapped into one.

These two examples show me that the urban poor (and even many farmer's sons) did not necessarily want land. They wanted money or basic economic support.

So who wanted land reform? Probably ironically conservative reformers who wanted Rome to be a country of yeoman farmer/soldiers supporting themselves and their country individually. These reforms mostly run afoul of the wealthy businessmen of the times and those businessmen made up the most entrenched members of the government and their wealthiest backers, hence they could convincingly claim that reformers were anti-establishment upstarts and link them to radical popular movements.

Finally, each reform and each reformer was different and they fail and succeed for different reasons. Tiberius Gracchus failed because he angered the Senate with his nepotistic commission (his brother and father-in-law were on it) and by going over their heads to fund the commission.