You asked why double shotgun was unable to handle higher pressure of big bore rounds.

Actually, pressure has little or nothing to do with bore size at all. For example, the British big bore cartridges, such as .470,, .500, even .577 and .600 are not considered high pressure, due to their large case size, while the smaller cased .458 Winchester is of higher pressure, due to that smaller case size. On the other hand, the little .270 is considered an intense cartridge, a high pressure cartridge, comparatively speaking. So, one cannot assume that big bore rifles are of high pressure, while smaller bore rifles are not. Pressure has no direct correlation with recoil either---one can have heavy recoil, with lower pressure, and little recoil with high pressure.
Double barreled rifles of all kinds, due to their design and bolting systems, are not suited to high pressure cartridges of any kind, though modern alloy steels have helped this situation considerably. Double barreled shotgun conversions are also obviously not suitable to high pressure cartridge conversions too, are not as large physically--not as much metal left in areas where needed, so one must be more conservative and use good judgment when converting them to low or medium pressure cartridges only; certainly never to high pressure cartridges. Chopper lump barrels were developed way prior to monoblock development, as a best method of construction, with far less likelihood of failure, but it seems likely now, that monoblocking is just as sound as is chopper lump barrel construction, as proven by numerous rifles built using monoblock system. Today, it is much more difficult, and more expensive, to make chopper lump barrels, rather than monoblocking, thus monoblocking being popular with gunmakers. Monoblocking is used frequently today for both double shotgun and double rifle manufacturing.