NCAA Division 1 tennis is very, very elite. A number of schools import their entire starting lineups from foreign countries. They do this because (1) There are only so many top-ranked American players, and (2) it's not against NCAA rules.
If you've never been highly ranked at the national or at least the sectional level, that may explain the coach's lack of responding to your inquiry. Or perhaps he/she doesn't like to write letters or emails, and so, many other young players are not getting a response, either.
Do not take the attitude that it's your favorite Division 1 school, or no college tennis at all. If you're a fairly strong player, there are probably many, many schools that would love to have you. Even if you're NOT a really strong player, there are still schools you can play for. Be flexible.
If you insist on going to a school where you will never start with their top six singles players or top doubles teams, then you are just asking for a heartache [I'm assuming tennis is important to you].
If you're a strong player, then you might very well be *much* happier at a Division 2 or 3 school where you can play #1 or #2. However, don't assume that you *will* play #1 at a Division 3 school, because you will find out that D-3 has a lot of strong players, too. With all the foreign players coming into the country, the overall skill level at D-2 and D-3 schools has gone up as well.
Do your homework as far as researching the schools that you're interested in. The coach is a very key person and will be a central figure in your life for at least a year, maybe 4 years. If you don't already have the USTA book below, get yourself a copy! Only $4.75 plus shipping :-) The knowledge contained inside it will certainly help you navigate the months and years ahead in your college tennis.
Note: Skill at handling a tennis racket does not necessarily translate into success later in life. In fact, IMHO, it's largely a useless skill as far as making money and/or helping others in some meaningful way. There are some who have been able to earn a good living in tennis, but they are NOT in the majority, IMO.
Sportsmanship might not be #1, but it's way ahead of whatever is in 2nd place.
Good luck with the struggle, hope you have some fun, win few matches, and graduate with a degree of some kind that will help you later in life :-)