There are a number of reasons. Among them (and I don't claim to have a definitive answer, just some insights as an alum):
-Bates was one of the first colleges in the country to be co-educational. While schools like Williams, Bowdoin and Amherst didn't accept women, who traditionally were outearned by men and did not leave as substantial monies to their colleges, until the 1970s, Bates has always accepted women and African American students.
-For much of its history Bates had a tradition of turning out "teachers and preachers", smart people who did good things in the wider world but who did not make a ton of money. In addition, a higher percentage of Bates students came from working and middle-class families than did their peers at most NESCAC schools.
-Bates was conservative with its investments during a critical time period when the markets were hot and other schools, already ahead, were able to realize higher investment gains.
-My opinion here, but I don't think that until recently Bates did a great job pushing its alumni for donations. Also, lacking frats, sororities or a big-time football program there just wasn't that "Rah-rah, gotta support the school" vibe to Bates. They've been more successful at bringing in alumni donations in recent years.
As Mindy noted, Bates is generous with financial aid. The school gives a high portion of its financial aid awards in the form of grants. For instance, according to the school's finaid page, among students receiving financial aid in fall 2011, the average grant was $35,089, and the average financial aid package (grant, loan and work) was $38,700. That means the average student had only $3,611 to make up per year in loans or work-study.
To answer the second part of your question, Bates doesn't have it's own golf course like Hamilton or Williams. Taconic Golf Club - Willipedia
Bates doesn't have a ski mountain like Middlebury or Dartmouth. Middlebury College Snow Bowl | Middlebury
The school has focussed on basics while still providing a beautiful and comfortable campus and excellent academics. I wouldn't worry about the impact of Bates' endowment on your education. They do quite well with what they have, although obviously they'd like more! Diversity and Inclusion | Bates College