A mass marketer needs to reach the masses, and to do it in many ways, simultaneously. The mass marketer needs retail outlets and fliers and a website and public relations and tv ads and more more more and then… bam… critical mass is reached and success occurs.
Best Buy is a mass marketer, but so are Microsoft and the Red Cross. Ubiquity, once achieved, brings them revenue, which advances the cycle and they reach scale.
The direct marketer, on the other hand, must get it right in the small. That pitch letter can be tested on 100 houses and if it gets a 2% response rate, then it can be mailed to 100,000 houses with confidence. That business-to-business sales pitch can be honed on one or two or three prospects, and then when it works, can be taught to dozens or hundreds of other salespeople.
The key distinction is when you know it's going to work. The mass marketer doesn't know until the end. The direct marketer knows in the beginning.
The mass marketer is betting on thousands of tiny cues, little clues, and unrecorded (but vital) conversations. The direct marketer is measuring conversion rates from the first day.
That's the reason we often default to acting like mass marketers. We're putting off the day of reckoning, betting on the miracle around the corner, spending our time and energy on the early steps without the downside of admitting failure to the boss.
Of course, just because it's our default doesn't mean it's right. Business to business marketing is almost always better if you treat it like direct marketing. Most websites that do conversion as well. Same with non-profit fundraising. As well as marketing goods and services to the bottom of the pyramid, people who live in villages where mass media and mass distribution are difficult and have little impact.
Get it right for ten people before you rush around scaling up to a thousand. It's far less romantic than spending money at the start, but it's the reliable, proven way to get to scale if you care enough to do the work.