TO LAY OR NOT TO LAY EGGS

Why lay eggs? So that you don't have to haul them around with you.
Why not lay eggs? Because they might be more vulnerable to predation and they might take longer to hatch in the cold soil.

What's the option? Retain the eggs within your body (ovoviviparous versus oviparous) so that you can keep them safer and use behavioral thermoregulation (shuttling in and out of the sun, orienting body toward sun, etc) to keep them warmer so that they develop faster and hatch sooner.
What's the problem with this option? The space the eggs take up and weight of the eggs may make the individual more susceptible to predation, not to mention the additional energy expenditures required to lug around the eggs.

Solution. All birds lay eggs (weight, after all, is critical to flight). But not all reptiles lay eggs. Around Tucson, lizards that live in the warmer desert habitats (e.g., Regal Horned Lizard and Desert Spiny Lizard) lay eggs (dump the weight and space) while the lizards in the higher elevations (e.g., Short-horned Lizard and Mountain Spiny Lizard) give birth to live young (to ensure the eggs stay warm enough).