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The [pre-AGA] Amiga:

* uses a planar frame buffer, stored in memory that is shared with the CPU (and which therefore reduces the CPU's speed);
* provides dual-playfield hardware scrolling, but the total bit fetch total per output byte doesn't stretch beyond five, so you end up with e.g. a 7 colour foreground and a four colour background;
* provides 8 hardware sprites, but with no DMA support the programmer is responsible for multiplexing them; but
* has the Copper for no-CPU cost raster racing; and
* has a blitter for quickly populating frame buffer contents.

Not all memory has to be shared with the CPU but Commodore being the company it was, the mainstream stock Amigas shipped with only shared memory.

Philosophically the Mega Drive is from a completely different school of thought:

* the VDP has 64kb of its own memory, which the CPU can access very slowly or else it can request a DMA transfer and yield the bus;
* from that memory the Mega Drive draws two tile-based play fields of 4bpp each, for which the programmer is permitted directly to supply per-line scrolling offsets;
* one of those play fields may optionally also display a non-scrolling window;
* it also supports a sprite layer with another 4bpp, for which it parses an 80-entry table to draw up to 20 16px-across sprites per line; and
* it has a Z80 entirely dedicated to audio, making that one less thing to do.

So unlike the Amiga, the Mega Drive is heavily optimised for 2d games of the era, providing native tile-based graphics and a large number of sprites that it can automatically multiplex.

Tiles and sprites are also great for forcing the programmer's hand: either the hardware can display what you want, and it'll work at the native refresh rate, or it can't. Unlike the blitter on the Amiga you can't decide you're going to run at 25fps rather than 50 in order to draw more.

There's also the advantage of the dedicated VDP RAM and the greater bandwidth the Mega Drive therefore enjoys, giving it two background layers of 4bpp graphics compared to the Amiga's 5bpp total — even before you start counting the much greater array of things you can do with the Mega Drive's sprites given that there's bandwidth for 320px worth of those per line.
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So while hardware can play a role, at the time of 16 bit systems of the mid to late 1980s it became less of a defining issue while budget became the ruling issue. Money defined the quality of programmers/companies to be hired and how much time they got to make the game great.

And that's where scale of a market comes into play. The market for Mega Drive games was at least a magnitude (10 times) larger than for Amiga games (*3). As a result game development fr the Mega Drive was based on way better funding and larger margins than anything done for the Amiga.
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*3 - The Mega Drive sold in the US during its first year about 500k units (plus about 400k in Japan), while the Amiga sold less than 100k worldwide. Over the 10 year lifetime of either machine the Amiga sold less than 5M units (worldwide, all models combines) and about 700k in the US. Sega alone sold more than 30 million units during the first run - 4 M units alone in the US during the first 4 years.
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