There are at least three major reasons why interest groups are increasing in number and power in our political system today.
One reason is that the legal restrictions on interest groups have been greatly reduced in the past decade or so. Perhaps the most important reduction in these restrictions came in the Citizens United case from 2010. In this case, the Supreme Court said that interest groups could air any advertisements that they wanted at any time so long as they did not coordinate with specific campaigns. In the past, interest groups could not air direct attacks on a specific candidate or appeals to vote for a candidate. They could not air political messages within 60 days of a general election. All of these restrictions were removed, giving interest groups much more power.
A second reason is that our government is bigger today than in the past. Our government does more things, which means that more people have to be concerned with what the government does. This leads them to form and join interest groups.
A final reason is that we now have easier communications. The internet and other modern technologies make it much easier for interest groups to reach out to potential members. It makes it much easier for them to raise money. It makes it much easier for them to get their message to people who might not have heard it in the time before the internet.
For all of these reasons, interest groups are getting to be more numerous and more powerful.