I personally do not invest in IOTA because it's market cap is already pretty high.
This is a really bad argument :d Do you not invest/trade in/for BTC?
It's a perfectly legitimate argument, it's one reason I have also not considered investing in IOTA, it has a big market cap already for something that is in development. It doesn't have a huge amount of room to grow in and so many investors may choose to look elsewhere.
market cap is big because all the coins are already mined, yet the value per coin is still low and has some headroom (from what I've read).
Also, since the amount of coins are a lot more than BTc it has much better scalability than BTC