Certainly easier and faster, with late Medieval technology. This has a very large effect on the price (the cost of the iron/steel often being less than 10% of the final price). From the 14th century onwards, cheap plate was cheaper than mail for this reason. (see Williams 2003, chapter 8.3, for some price data).

There are some technical difficulties with making plate cuirasses. In particular, for making an iron/steel cuirass, it can be difficult to heat such a large workpiece to forging temperature and then handle it (and it will cool down quickly). A large forge furnace is needed, which will be fuel-hungry. This is better suited to a large workshop, where multiple workers can use it, than a small one-man workshop. Turning a lump of iron/steel into a relatively thin sheet is also a non-trivial task, and one that can be eased with mechanisation (e.g., power hammers, which were in use in Europe from at least the 12th century). Plate armour benefits a lot from large-scale manufacture and industrialisation.

(Since bronze is cold-forged, there was no problem of heating the workpiece for forging when making ancient bronze plate. The workpiece did still need to be heated for quenching to relieve work-hardening.)

A plate cuirass would also undoubtedly provide much more protection for your vitals from pretty much any weapons you'd encounter in a fight.

Why? Given equal weights per unit area, why would plate protect much better than mail? Plate has the advantage of rigidity for protection against blunt impact, and mail has the advantage of mobility for protection against arrows (energy that gets used for moving the armour isn't available for penetrating the armour). Plate has the advantage of better deflection of arrows if they hit away from square-on.

Thick plate will certainly protect better than lightweight mail, but heavy mail is quite protective (and was often used in a textile-mail-textile sandwich).

Lastly, a smartly designed cuirass should not significantly inhibit the movement of its wearer, so any movement advantage of wearing just mail seems like it would be negligible to me.

The supposed "movement advantage" of mail will only result from comparing lightweight mail with heavy plate, or with plate that doesn't fit the wearer. But it's worth considering what the functional cost is for making plate easy to move in. A plate cuirass avoids inhibiting movement by protecting less area than a mail shirt. The cuirass needs to leave enough clearance at the neck and armpit to not hinder movement, and only protects to the waist (the typical cuirass stops at about elbow level), to allow bending over. A cuirass alone provides much less protection than a mail shirt when the area covered is taken into account. A cuirass + gorget + arms + tassets will provide similar coverage, but will have gaps in the armpits and along the inside of the arms, often uncovered buttocks, and more gaps around the groin and thighs. Essentially, plate has gaps that mail doesn't have. It is, of course, possible to combine the two, for example by wearing a plate breastplate over mail for improved protection (and such breastplates tend to be of modest weight, since they don't need to be thick enough for standalone protection, perhaps 1.6mm thick rather than 2-3mm thick), or by using mail to cover the gaps in the plate.

While mail can be reinforced by wearing textile armour under and on top of it (as was done, historically), a plate breastplate (or coat-of-plates) will provide more increased protection for less weight than added textile armour. Not including the issue of gaps, thicker plate will provide better protection for the weight than plate + mail (the energy needed to penetrate plate increases faster than linearly, to the power of 1.6 (Williams, 2003, pg 928). As the cost of plate relative to mail dropped, switching to all-plate (except for mail to cover gaps) appears to have been a cost-effective and weight-effective choice for improved protection (the competing technologies being plate, coat-of-plates, or lamellar, or more mail, or textile amour worn over mail or simply thicker and heavier mail). Noting that mail needs small rings to keep arrows out, thick heavy mail will be harder to make than light mail - this probably contributed to the use of additional armour being worn over mail rather than simply using thicker heavier mail.

What was this extra armour on top of mail used for? To stop arrows from high draw weight bows and to stop bolts from powerful crossbows. Textile-mail-textile appears to have worked well against arrows and crossbow bolts, but was heavy. Mail + coat-of-plates (or + lamellar) worked well, but was heavy. Factors that might have favoured switching to plate might have been increasingly powerful crossbows (needing further increased protection) or an increase in fully-armoured soldiers fighting on foot (driving reduction in weight). Guns certainly favour thick plate.

As for why ancient plate disappeared in favour of mail: probably cost gaps, and the lack of high-power crossbows and guns on the battlefield. Helmets (including bronze helmets) continued to be used, so the technology to make plate armour pieces still existed (and for helmets, the extra protection against blunt impact provided by a rigid material is useful). When labour was cheap, and production small-scale, mail was cheaper. With late Medieval rises in wages, and increased industrialisation, mail shifted from cheaper to more expensive, and plate was favoured by cost.

The need for increased torso protection is also greatly reduced by shields. Shields protect the chest quite well. While there is a chicken-and-egg issue with the widespread adoption of two-handed weapons and torso armour that allows shields to be abandoned, if one-handed weapons and shields are used, the shield will provide torso protection.

Reference:

Alan R. Williams, The Knight and the Blast Furnace: A History of the Metallurgy of Armour in the Middle Ages & the Early Modern Period, Brill, 2003. https://books.google.com/books/about/The_Knight_and_the_Blast_Furnace.html?id=GpVbnsqAzxIC