why am I getting low credit limit?

Credit card companies normally offer low credit limits in case the borrower has outstanding debts or has a bad repayment history. As you've paid your debts on time, the credit card company should have offered you a better credit limit. You should contact your credit card company and check out why they won't be giving you a higher credit limit. They will be the best to help you in this matter.

Not necessarily. What's your debt to income ratio? These other cards you have...are they maxed out? Are you only paying minimum payments? You available credit is based on your risk level. If you're deemed high risk due to any of these factors then the low limit makes perfect sense. It could also be the company's policy to start customers with a low limit and increase it as they prove their credit worthiness.

I agree. Timely payment of credit is proven, and not usually based on financial status. In fact, the risk analysis done by FICO does not even consider your income or total debt. It is based primarily on their risk analysis of proven permormance in actually making timely payment, regardless of your ability to have paid timely.
As your CL goes up, so does their potential risk in granting you that level of credit. Until you have shown the ability to manage that level of authorized debt, they want to put limits on it until you have proven your reliability to pay timely, regardless of financial ability to have done so.